While I was testing something on my flat world and spawning Withers, the game has crashed when I have half a heart when getting killed by it.
Edit: It also applies to other potion/status effects that kill you eg. Harming, Fatal Poison.
Steps to Reproduce:
Use an effect that damage you Example: wither, harming, fatal poison
Let it completely deplete your health
After that the game crashes
Also affects Instant Damage and Fatal Poison according to MCPE-153174.