Multiplayer bug. I play the game and my little brother connect lan for me.
A player goes over to the nether or the end, and as soon as he returns to the normal world, the spawner stops or disappears from it the mob. And only a game/server restart will solve it.
Thank you for your report!
We're tracking this issue in MCPE-142285, so this ticket is being resolved and linked as a duplicate.
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