Wither bosses can break obsidian and go through bed rock

Wither bosses can break obsidian and go through bed rock
*What I expected to happen was it staying
what happen here
trap in obsidian
*What actually happened was it broke it when it was going through it
It also did not leave the blocks for me to pick up

Steps to Reproduce:
1. spawn the wither boss in a obsidian cage
2. have it follow you
3. the wither boss breakes the obsidian