If you close a world and reopen it while there's a wither nearby, it will do its spawning animation again. The same thing happens if you move far away from it until it stops rendering and then come back close to it.
Linked issues
clones 1
is duplicated by 7
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I would like to add that at least for me when the animation replays below half health it will visually remove the wither's armor that repels arrows. the armor only disappears after the animation replays so think that's the cause.
Can confirm. This appears to be a regression of MCPE-32415.