(mod edit: happens in 5.18 and later)
I am selecting a bunch of notes in the ME and applying the action Edit: Note velocity -10. This is not showing up in the undo history. If I then do an undo, the edit gets undone. But if I apply the action multiple times, and then try to successively undo, the first undo command reverts back to the initial values before any of the actions were run.
That is somewhat similar to how "Move notes down one semitone works," where doing that action rapidly multiple times will create as a single undo point, but where doing it slowly registers as multiple undo events. But with the velocity action, each time it's run is consolidated into one undo point, but none of the times its run are actually showing up in the undo list. (This is not a custom action where I've consolidated undo points.)
This licecap sort of shows it.