If you have a spawnpoint in an unloaded chunk above y level 256 such as a bed, if you respawn while the chunks are unloaded the chunks will simply bug out and not load in, making you fall through the world and softlocking you. you will then time out and will be unable to rejoin the server.
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We're tracking this issue in MC-214808, so this ticket is being resolved and linked as a duplicate.
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