If you spawn the wither boss and inflict damage to it, the boss destroys the blocks it is in contact with at the time it took damage. In the past, these blocks have disappeared without a trace, but now when they are destroyed, they drop as items on the ground, and if you're fighting the wither underground where it can't fly up and away, the dropped items make the game extremely laggy until enough of them are picked up.
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However, this issue has been closed as a Duplicate of MCPE-21854
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