Yeah, but I rather have warded... And lemme explain the whole thing see if that makes more sense:
The wither has 3 different attacks, 1 the black projectiles, that damage the player, 2 the block busters, the blue projectiles that destroy every block in his path, including obsidian excluding bedrock, 3 the body attack, when he takes damage and is touching a block, he will try to break it (besides bedrock, is hardcoded into his class) and he will succeed, in every block, now, how come when he breaks a warded block it regenerates and he scapes, but with a forcefield not?, both regenerate, where is the problem here? Is because (at least im 90% sure) warded blocks count as solid blocks, even the glass, and the forcefield counts as a transparent block, now, as we now, steve, or any mob, does not take damage from being inside a transparent block, so, since when the wither takes damage and tries to break blocks around him, he will break them, since forcefields are transparent, they will just regenerate and move the wither back to where he was, but since warded are solid,the wither will take damage inside them and try to break them again, and so he will succeed and scape (since he is already more than half way trough the block, unlike forcefields, that he enters just a little bit)