We get a spike of reports of Valentine's revives (Kill Joy and Last Hope) not working during the Ms. Fortune Prize Fight since it features a lot of PRECISION. PRECISION will prevent defensive Signature Abilities from firing for one hit.
If you kill a Last Hope with a PRECISION hit, she will not revive. Is it possible this is what you experienced? We'll need to see a video of the bug in action if you think it might be something else.
Sorry for the inconvenience, and thanks for the report!