The bug

Moved blocks (piston_extension) 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"}]}
   

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

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

3. Punch the wither to make it break the block