In 22w15a, allays were given a natural health regeneration of 2 health points per second, which (possibly intentionally) makes them unable to die to most types of damage, including fire, cactus, drowning, freezing, suffocating, punching. etc.
How to reproduce:
Trap an allay somewhere where they take passive damage of 1 or 2 health per second.
Observe.
this seems to be WAI as the regeneration was added to stop allays from dying to environmental and accidental damage sources as per slicedlime's snapshot video.