Sometimes if the wither storm evolves to phase 4 at the wrong moment it can get stuck with the terrain and basically never move from it.

A easy solution is to make wither storm phase 4 able to break the blocks that are touching it's hitbox, at least to make it able to get to the sky.

The wither storm will now (in the next update) explode blocks when colliding with them. The only thing is that this doesn't work with water, however I don't think there will be much cases of this happening/