It's a pretty poor user experience when I have someone showing me in their plan that they're trying to change a finish date and the task bounces back to whatever date was previously there.
The fix is to change the constraint to "Fixed Dates" - hardly a novice user's first port of call.
Automatically bouncing the date back and the system saying nothing to explain it is very poor indeed. I suggest to add some kind of message - even if just a non-modal banner at the top.
