Wither can teleport inside blocks and get stuck there
Here's the bug: Usually this happens when there are non-undead mobs nearby and it's not targeting the player, but this can be annoying in creative mode, or you just spawned in the wither and it teleported away to attack some other animals, then gets itself stuck inside blocks. I suggest a change where the Wither can still teleport, but not inside blocks, or make it to where it has the ability to break any blocks it runs into, without needing to attack it first.
This is just another idea, but is it possible you can make the wither have abilities similar to the MCPE Wither, like when it charges at the target when it's at half-health?