When Loading into world, the block explosion will occur without being attacked.

 To reproduce:

Step 1: trap the wither(preferably in bedrock)

step 2: put a non-blast-resistant block in the radius of the withers spawn explosion after spawn explosion has occurred.

Step 3: unload the chunk that hold the wither or save and exit. 

step 4:reload the game/reload chunks, and blocks should be broken. Blocks only become broken in the coded break spots of the with explosion.