The Wither performs a block breaking attack around him in a 4 wide, 4 long, 6 tall area around him when damaged. He will also perform it along with the spawn explosion on intial spawn. The reason I think this is because in the 1.16 update, the wither was changed so that it no longer broke obsidian when spawned in, but this also came along with another bug. the wither block breaking attack and block breaking dash, which just the block breaking attack prolonged and moved, no longer break obsidian. This is because to fix “the spawn explosion breaking obsidian” the changed the block breaking attack that occurred with the spawn explosion since the actual spawn explosion never did blow up obsidian. This can be proven and reproduced by:

Step 1: trap the wither in some sort of cage, preferably bedrock. 

step 2: figure out the block breaking attack area(should be 1 block belies him up to as high as two blocks above him, a four by four square centered around him, him being at the northwest center block.)

step3: after determining on where he will spawn and the potential break spot when he will spawn in, place some cobble within that area, then place some cobble barely outside the area(would recommend one block outside the area. The time you put cobble within the area, you should hear the block breaking attack sound and the cobblestone should be destroyed and drop as an item. The second time you, when cobblestone is outside of the area, the cobblestone should have broken but you shouldn’t have heard the block breaking attack sound(block breaking attack also occurs when wither is damaged).

step4: repeat step 3 but instead of cobblestone block, use water sources. Water source should break within the block breaking attack area when the wither is spawned in. Outside the block breaking attack are but within the explosion water should not break and no block breaking attack sound should play. 

Block breaking attack should break every block but bedrock.

spawn explosion should not break obsidian.