Basically, as the title suggests. The Wither Boss mob will destroy the room, or at the very least, knock the wall blocks to the ground. While this does not cause the game to crash, it is likely not intended functionality.

Easy to replicate. Spawn mob inside compact machine.

Are you using the latest version of the mod? That should be fixed already.

Assuming that the latest version is still 1.9, then yes. I updated manually to verify. However, I am not discounting the possibility that another mod in the Modsauce pack has changed Wither functionality either, now that I think about it.

This seems to be the case. The "normal" Wither actually checks canEntityDestroy and hence does not break the blocks.
I have no idea though which of the mods modifies/replaces the wither though.