Vivaldi tends to use their own problematic builtin notifications if Vivaldi starts before the desktop's notifications system, instead of using the desktop's native notifications. This script should fix the problem by forcing Vivaldi to start once the desktop notifications system is ready. Just make this script starts in the "applications on boot…

start-browser-before-notifications.sh

#!/bin/sh

# wait for plasmashell to response on D-Bus
while ! qdbus org.kde.plasmashell /PlasmaShell > /dev/null 2>&1; do
    sleep 1
done

# wait for the notifications service
while ! qdbus org.freedesktop.Notifications /org/freedesktop/Notifications > /dev/null 2>&1; do
    sleep 1
done

# launch browser (edit if needed)
exec /usr/bin/vivaldi-stable --enable-speech-synthesis --enable-features=UseOzonePlatform --use-gl=egl --flag-switches-begin --ozone-platform-hint=wayland --flag-switches-end --ozone-platform=wayland %U &