If you try to log into a server that uses a server resource pack while standing in an illegal position (ie. on a boat) you are immobilized in seeming mid-air until the server pack finishes loading, and it promptly dropkicks you from the server for flying.
Workaround: Disable server resource pack and rejoin
I'm creating this bug because mandatory server resource packs are on the roadmap and they would lock the player out of the server without the intervention of another player or server admin to move the boat/player
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We're tracking this issue in MC-83938, so this ticket is being resolved and linked as a duplicate.
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