When a user does undo a stroke krita sometimes randomly stops updating the titles, making it seem like undo is not working, however if we try to draw over these strokes (which are still visible) we get new stroke and artifacts the portion your new pixels are laid out get cleared. 

There is also a problem of redo not working, so you are left in state of broken undo and new strokes.

This is making git master unusable and the bug is random and really not consistent so it is hard to pinpoint anything.