Since bash 5.1, PROMPT_COMMAND can be an array.
That's nice, as it's easier to robustly operate on an array on per element basis than on a delimited string to that effect.
Things that add to the variable can coexist and just operate on it as they would normally do, whether they treat it as an array or a scalar, without doing extraneous checks.
The first gotcha related to this is that operating on an array variable as if it was a scalar operates on the array's first element.