The bug

Moved blocks (moving_piston) should be breakable by withers or it should at least be determined by the moved block type. It was a mistake (by marcono1234) to include it in MC-113429 since the actual problem was MC-58556.

How to reproduce

1. Summon a wither

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

1. Place an piston extension block in the area in the wither can break

   /execute at @e[type=wither,c=1] run /setblock ~ ~1 ~ moving_piston
   

1. Punch the wither to make it break the block