Wither breaks obsidian when spawned

Wither breaks obsidian, 

Steps to Reproduce:

1. Made a obsidian cage for wither
2. Spawn wither inside obsidian cage and close it up with 2 more blocks of obsidian,
3. Wither spawns and does startup explosive and breaks obsidian,

Observed Results:
Wither spawns and breaks obsidian, 

Expected Results:
Wither spawns and does startup explosive but doesn't break obsidian,

Screenshots/Videos attached: [please attach an image or short video]

Notes: this bug is also in 1.14.1.3 beta, Xbox One Bedrock,