Wither skeletons are harmed, and can die from the wither effect on bedrock edition, usually withers are immune to the wither effect.. Because they are withers xd

This affects game-play when fighting a wither boss on hard, they will summon 3 (should be 4) wither skeletons when it reaches half health. Those wither skeletons will immediately get the wither effect, and try to attack the wither boss. Making them basically pointless as they die and rarely attack the player.

You can see more about this bug in my recent bugrock episode, here:
MCPE-34497