Hostile mobs that manage to kill the player disappear after the player respawns.
This also happens if you die in another dimension, the attacker disappears, only the bosses are still on the site where the player dies.
Linked issues
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We're tracking this issue in MCPE-94764, so this ticket is being resolved and linked as a duplicate.
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Hostile mobs despawn when they are certain distance away from player (i.e. more then 128 blocks).