Okay so I've been researching this movement flickering issue. I compiled some client network sniffs of the movement data and flattened them out. Each time I observed a flicker used a macro that `/say here` so I could see if there are any strange position data sent to the client from our core. Not sure if there is anything strange happening.

Flicker Movement - Client Network Data

Flicker Movement Client Network Data:
  * -flicker = the moment I observed a flicker in game
  
--[1]----------------------

X: -602.9882 Y: -4252.852 Z: 38.95627 O: 6.067197 (MSG_MOVE_SET_FACING, Time: 225716) 
X: -602.6502 Y: -4252.942 Z: 38.95627 O: 5.925826 (MSG_MOVE_SET_FACING, Time: 225766) (50ms)
X: -602.4381 Y: -4253.033 Z: 38.95627 O: 5.772674 (MSG_MOVE_SET_FACING, Time: 225799) (33ms)
X: -602.2281 Y: -4253.159 Z: 38.95627 O: 5.654864 (MSG_MOVE_SET_FACING, Time: 225834) (35ms)
X: -601.9659 Y: -4253.368 Z: 38.95627 O: 5.525274 (MSG_MOVE_SET_FACING, Time: 225882) (500ms)
X: -599.5956 Y: -4255.943 Z: 38.95627 O: 5.450661 (MSG_MOVE_HEARTBEAT,  Time: 226382)┐(500ms)
>> flicker                                                                           |
X: -597.2401 Y: -4258.532 Z: 38.95627 O: 5.450661 (MSG_MOVE_HEARTBEAT,  Time: 226882)┘(500ms)
X: -594.8851 Y: -4261.122 Z: 38.95627 O: 5.442807 (MSG_MOVE_HEARTBEAT,  Time: 227382) (500ms) 
X: -594.8118 Y: -4261.215 Z: 38.95627 O: 5.238604 (MSG_MOVE_SET_FACING, Time: 227399) (17ms)
X: -594.752  Y: -4261.318 Z: 38.95627 O: 5.085452 (MSG_MOVE_SET_FACING, Time: 227416) (17ms)
X: -594.7112 Y: -4261.423 Z: 38.95627 O: 4.940153 (MSG_MOVE_SET_FACING, Time: 227432) (16ms)

--[2]----------------------

X: -607.4963 Y: -4258.504 Z: 38.95602 O: 2.536835 (MSG_MOVE_SET_FACING, Time: 231768)
X: -607.6604 Y: -4258.397 Z: 38.95602 O: 2.646791 (MSG_MOVE_SET_FACING, Time: 231796) (28ms)
X: -607.8738 Y: -4258.292 Z: 38.95602 O: 2.784236 (MSG_MOVE_SET_FACING, Time: 231830) (34ms)
X: -608.207  Y: -4258.186 Z: 38.95602 O: 2.902046 (MSG_MOVE_SET_FACING, Time: 231880) (50ms)
X: -608.6603 Y: -4258.099 Z: 38.95602 O: 3.03949  (MSG_MOVE_SET_FACING, Time: 231946) (66ms)
X: -609.1284 Y: -4258.074 Z: 38.95602 O: 3.153373 (MSG_MOVE_SET_FACING, Time: 232013)┐(67ms)
>> flicker                                                                           |
X: -612.6124 Y: -4258.397 Z: 38.95602 O: 3.243694 (MSG_MOVE_HEARTBEAT,  Time: 232513)┘(500ms)
X: -613.4184 Y: -4258.493 Z: 38.95602 O: 3.455752 (MSG_MOVE_SET_FACING, Time: 232629) (116ms)
X: -613.5316 Y: -4258.529 Z: 38.95602 O: 3.628539 (MSG_MOVE_SET_FACING, Time: 232646) (17ms)
X: -613.4184 Y: -4258.493 Z: 38.95602 O: 3.455752 (MSG_MOVE_SET_FACING, Time: 232663) (17ms)
X: -613.5316 Y: -4258.529 Z: 38.95602 O: 3.628539 (MSG_MOVE_SET_FACING, Time: 232680) (17ms)

--[3]----------------------

X: -602.4247 Y: -4258.606 Z: 38.95483 O: 2.348337 (MSG_MOVE_SET_FACING, Time: 240563)
X: -602.5919 Y: -4258.447 Z: 38.95483 O: 2.454365 (MSG_MOVE_SET_FACING, Time: 240596) (33ms)
X: -603.4421 Y: -4257.796 Z: 38.95483 O: 2.564321 (MSG_MOVE_SET_FACING, Time: 240749) (153ms)
X: -603.828  Y: -4257.569 Z: 38.95483 O: 2.678204 (MSG_MOVE_SET_FACING, Time: 240813) (64ms)
X: -604.2617 Y: -4257.373 Z: 38.95483 O: 2.796014 (MSG_MOVE_SET_FACING, Time: 240881) (68ms)
X: -604.5886 Y: -4257.27  Z: 38.95483 O: 2.90597  (MSG_MOVE_SET_FACING, Time: 240930)┐(49ms)
>> flicker                                                                           |
X: -605.97   Y: -4257.046 Z: 38.95483 O: 3.019852 (MSG_MOVE_SET_FACING, Time: 241130)┘(200ms)
X: -606.3185 Y: -4257.018 Z: 38.95483 O: 3.141589 (MSG_MOVE_SET_FACING, Time: 241180) (50ms)
X: -608.5349 Y: -4257.123 Z: 38.95483 O: 3.251545 (MSG_MOVE_SET_FACING, Time: 241497) (317ms)
X: -608.6462 Y: -4257.135 Z: 38.95483 O: 3.361501 (MSG_MOVE_SET_FACING, Time: 241513) (16ms)
X: -608.7692 Y: -4257.162 Z: 38.95483 O: 3.471457 (MSG_MOVE_SET_FACING, Time: 241531) (18ms)

--[4]----------------------

X: -610.7586 Y: -4254.211 Z: 38.95638 O: 5.352482 (MSG_MOVE_SET_FACING, Time: 247497)
X: -610.626  Y: -4254.4   Z: 38.95638 O: 5.242527 (MSG_MOVE_SET_FACING, Time: 247530) (33ms)
X: -610.515  Y: -4254.602 Z: 38.95638 O: 5.124717 (MSG_MOVE_SET_FACING, Time: 247563) (33ms)
X: -610.4293 Y: -4254.816 Z: 38.95638 O: 5.010835 (MSG_MOVE_SET_FACING, Time: 247596) (33ms)
X: -609.4766 Y: -4258.184 Z: 38.95638 O: 4.987273 (MSG_MOVE_HEARTBEAT,  Time: 248096) (500ms)
X: -608.5266 Y: -4261.553 Z: 38.95638 O: 4.987273 (MSG_MOVE_HEARTBEAT,  Time: 248596)┐(500ms)
>> flicker                                                                           |
X: -607.5766 Y: -4264.921 Z: 38.95638 O: 4.987273 (MSG_MOVE_HEARTBEAT,  Time: 249096)┘(500ms)
X: -607.4672 Y: -4265.261 Z: 38.95638 O: 5.215038 (MSG_MOVE_SET_FACING, Time: 249147) (51ms)
X: -607.4099 Y: -4265.365 Z: 38.95638 O: 5.391753 (MSG_MOVE_SET_FACING, Time: 249164) (17ms)
X: -607.3395 Y: -4265.452 Z: 38.95638 O: 5.576321 (MSG_MOVE_SET_FACING, Time: 249180) (16ms)
X: -607.249  Y: -4265.529 Z: 38.95638 O: 5.768744 (MSG_MOVE_SET_FACING, Time: 249197) (17ms)

--[5]----------------------

X: -593.2695 Y: -4257.595 Z: 38.95638 O: 1.727869 (MSG_MOVE_SET_FACING, Time: 251563)
X: -593.3164 Y: -4257.362 Z: 38.95638 O: 1.89673  (MSG_MOVE_SET_FACING, Time: 251597) (34ms)
X: -593.3987 Y: -4257.146 Z: 38.95638 O: 2.045956 (MSG_MOVE_SET_FACING, Time: 251630) (33ms)
X: -593.5101 Y: -4256.944 Z: 38.95638 O: 2.148057 (MSG_MOVE_SET_FACING, Time: 251663) (33ms)
X: -593.714  Y: -4256.66  Z: 38.95638 O: 2.277648 (MSG_MOVE_SET_FACING, Time: 251713) (50ms)
X: -593.9568 Y: -4256.399 Z: 38.95638 O: 2.395458 (MSG_MOVE_SET_FACING, Time: 251764)┐(51ms)
>> flicker                                                                           |
X: -596.5666 Y: -4254.251 Z: 38.95638 O: 2.195181 (MSG_MOVE_SET_FACING, Time: 252247)┘(483ms)
X: -596.6321 Y: -4254.16  Z: 38.95638 O: 2.042029 (MSG_MOVE_SET_FACING, Time: 252263) (16ms)
X: -596.6861 Y: -4254.054 Z: 38.95638 O: 1.884949 (MSG_MOVE_SET_FACING, Time: 252280) (17ms)
X: -596.7228 Y: -4253.941 Z: 38.95638 O: 1.767139 (MSG_MOVE_SET_FACING, Time: 252297) (17ms)
X: -596.759  Y: -4253.706 Z: 38.95638 O: 1.53152  (MSG_MOVE_SET_FACING, Time: 252331) (34ms)

--[6]----------------------

X: -610.38   Y: -4263.606 Z: 38.9547 O: 5.537049 (MSG_MOVE_SET_FACING, Time: 260551)
X: -610.22   Y: -4263.743 Z: 38.9547 O: 5.639151 (MSG_MOVE_SET_FACING, Time: 260581) (30ms)
X: -609.1803 Y: -4264.491 Z: 38.9547 O: 5.808011 (MSG_MOVE_SET_FACING, Time: 260764) (183ms)
X: -608.9709 Y: -4264.588 Z: 38.9547 O: 5.957237 (MSG_MOVE_SET_FACING, Time: 260797) (33ms)
X: -608.6345 Y: -4264.684 Z: 38.9547 O: 6.075047 (MSG_MOVE_SET_FACING, Time: 260847) (50ms)
X: -608.1726 Y: -4264.763 Z: 38.9547 O: 6.185002 (MSG_MOVE_SET_FACING, Time: 260914)┐(67ms)
>> flicker                                                                          |
X: -604.6837 Y: -4265.04  Z: 38.9547 O: 6.204637 (MSG_MOVE_HEARTBEAT,  Time: 261386)┘(472ms)
X: -603.7559 Y: -4265.118 Z: 38.9547 O: 6.047557 (MSG_MOVE_SET_FACING, Time: 261519) (133ms)
X: -603.6402 Y: -4265.146 Z: 38.9547 O: 5.878697 (MSG_MOVE_SET_FACING, Time: 261536) (17ms)
X: -603.5308 Y: -4265.192 Z: 38.9547 O: 5.709836 (MSG_MOVE_SET_FACING, Time: 261553) (17ms)
X: -603.4367 Y: -4265.253 Z: 38.9547 O: 5.572392 (MSG_MOVE_SET_FACING, Time: 261569) (16ms)

Key Observations:

• There are time spikes of around 200ms to 500ms when the "flicker" occurs.
• Predominantly observed when moving the players orientation while moving forward (left <-> right, opcode: MSG_MOVE_SET_FACING).
• Most of the flickers contain a jump in their X coordinate significantly larger than the previous trends, the same can be seen with the Y coordinate in some cases.

Footage:

General Notes:
2 minutes of running around shows flooding of MSG_MOVE_SET_FACING

--------starting metrics:-------
--------printing metrics:-------
 00000000000000DA: called 472 times
 [0] 0ms
 [1] 0ms
 [2] 0ms
 [3] 0ms
 [4] 0ms
 [5] 0ms

First movement packets show a high Time (probably our own movement):

ClientToServer: CMSG_TIME_SYNC_RESPONSE (0x0391) Length: 8 ConnIdx: 0 Time: 06/14/2019 18:41:56.000 Number: 8
Counter: 1
Ticks: 46150828

ClientToServer: MSG_MOVE_SET_FACING (0x00DA) Length: 29 ConnIdx: 0 Time: 06/14/2019 18:42:29.000 Number: 67
Movement Flags: 0 (None)
Extra Movement Flags: 0 (None)
Time: 46183195 << high
Position: X: -616.845 Y: -4253.15 Z: 38.933
Orientation: 6.271401
Fall Time: 0

ClientToServer: MSG_MOVE_SET_FACING (0x00DA) Length: 29 ConnIdx: 0 Time: 06/14/2019 18:42:29.000 Number: 68
Movement Flags: 0 (None)
Extra Movement Flags: 0 (None)
Time: 46183245 << high
Position: X: -616.845 Y: -4253.15 Z: 38.933
Orientation: 6.165372
Fall Time: 0

Then exhibiting movement from the other player the Time is a lot smaller:

ServerToClient: MSG_MOVE_FALL_LAND (0x00C9) Length: 31 ConnIdx: 0 Time: 06/14/2019 18:42:56.000 Number: 99
Guid: Full: 0x0000000F Type: Player Low: 15 Name: Rer
Movement Flags: 0 (None)
Extra Movement Flags: 0 (None)
Time: 167517 << lower
Position: X: -618.518 Y: -4251.67 Z: 38.718
Orientation: 0
Fall Time: 0

ServerToClient: SMSG_COMPRESSED_UPDATE_OBJECT (0x01F6) Length: 22 ConnIdx: 0 Time: 06/14/2019 18:42:56.000 Number: 100
Count: 1
Has Transport: false
[0] UpdateType: Values
[0] GUID: Full: 0x0000000F Type: Player Low: 15 Name: Rer

ServerToClient: MSG_MOVE_SET_FACING (0x00DA) Length: 31 ConnIdx: 0 Time: 06/14/2019 18:42:56.000 Number: 102
Guid: Full: 0x0000000F Type: Player Low: 15 Name: Rer
Movement Flags: 0 (None)
Extra Movement Flags: 0 (None)
Time: 168432 << lower
Position: X: -618.518 Y: -4251.67 Z: 38.718
Orientation: 6.275331
Fall Time: 0

ServerToClient: MSG_MOVE_START_STRAFE_RIGHT (0x00B9) Length: 31 ConnIdx: 0 Time: 06/14/2019 18:42:56.000 Number: 103
Guid: Full: 0x0000000F Type: Player Low: 15 Name: Rer
Movement Flags: 8 (StrafeRight)
Extra Movement Flags: 0 (None)
Time: 168449 << lower
Position: X: -618.518 Y: -4251.67 Z: 38.718
Orientation: 6.228208
Fall Time: 0

Further Tests:
I tested to see if the MSG_MOVE_SET_FACING packets observed on the client match the number of calls to the server, checking if they match 1:1 and they do. 100 packets from the client result in 100 calls to and from the server. (can post results later)

Did you figure out what the issue was?