Game Mode: both
I think it's important to solve, because bedrock is the only material (exept air) which can't be destroyed by the wither.

The wither spawning point is surrounded by a dome (a big half of a sphere), which is made from bedrock are very deep in the earth, only a few blocks from the void. Between the ground of the dome and the void is only a 1-block thick floor. At some places, there is glass which shall be destroyed from the wither, made to let the player fall into the void, if he isn't careful enough in this final battle.

The central issue is, that, if the wither has spawned, it is hovering to the very bottom of the dome and gets stuck there. Forever.
I can destroy a few blocks in which the wither is stucking, to let it fly trough the floor into the void, but the boss disappears a few moments later.
After a short time the boss appears again at its spawning point and repeats everything. And then, it's a ever lasting process of respawn and disappear.

Some things I noticed:

• If I attack the spawning point while he isn't actually there, the sounds are played. The wither takes no damage, but the sounds are played. Also, it appears again in a much shorter time.
• If I try to let him spawn somewhere else at the ground for example, he is permanently flying against the wall.