The bug

desync on server with spectator and riding things. see clip: https://clips.twitch.tv/BoredWonderfulSpiderTTours-iERoDWqOQq6or6fi

How to reproduce

Note: The exact conditions required to replicate this are unknown.

1. Disable the spectatorsGenerateChunks gamerule:

   /gamerule spectatorsGenerateChunks false

2. Ensure you are not in any force loaded chunks or spawn chunks
3. Ride a boat
4. Switch to spectator mode
   → You remain in the boat, and are still able to move the boat

Expected result

The player should be dismounted from the boat when they enter Spectator mode.