The wither can break the blocks listed above with its explosion, even though they have the same blast resistance as obsidian AND many of these blocks can be made from obsidian, while obsidian is not broken by these explosions.

To reproduce:

step 1: make a small bedrock room

step 2: put some ancient debris in there

step 3: spawn the wither

step 4: watch as the spawn explosion breaks the ancient debris(or other block listed above)

step 5:place more of (chosen “blast resistant”) block)

step 6: damage wither when he is close to your chosen block

step 7: watch him explode the block again

Edit: I have discovered that this is not a bug.