The bug

The wither boss can break piston extensions and end gateways. For both blocks it displays the missing texture (purple and black) breaking particles.

How to reproduce

1. Summon a wither

   /summon armor_stand ~ ~ ~ {Invulnerable:1b,Passengers:[{id:"wither"}]}
   

2. Place an affected block in the area in the wither can break

   /execute @e[type=wither,c=1] ~ ~ ~ /setblock ~ ~1 ~ end_gateway
   

3. Punch the wither to make it break the block

Fix

Add piston_extension and end_gateway to net.minecraft.entity.boss.EntityWither.canDestroyBlock(Block blockIn).