First, the starting animation is broken. Only the middle head is spinning rather than the whole body.
Second, in the first phase of the boss, the wither most of the time just stands still and suddenly falls down and then slowly flies up to its original place again from time to time. It's supposed to fly around way more.
Thank you for your report!
We're tracking this issue in MCPE-64537, so this ticket is being resolved and linked as a duplicate.
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