Expectation: Mobs holding Totems of Undying can survive death one time and will regenerate lost health after cheating death.

Reality: While mobs can cheat death, they will not be given the regeneration effect, instead being left with 2 points of health (estimated by them dying from two punches after being given a few minutes to heal)

To reproduce, please /summon a mob with a totem of undying in hand, then kill it once so that it will use it. Wait a few minutes for the regeneration effect to maybe work, then punch it twice and see if it dies.