Ender IO 1.8.9-3.0.0.17 (I know you've updated since, but did not see fix in changelog)

Reinforced Obsidian is not witherproof.

Built a 2-thick chamber to spawn wither, with a 1 block gap to place skulls. Wither spawns and destroys chamber (blocks not completely destroyed, as they drop as entities). It seems the projectiles do the damage and not the initial explosion, as the bottom layer is intact.

Not sure if this is intended behaviour (blocks not being projectile proof)?