The bug
I, the LAN host, was piloting a happy ghast with my friend on my LAN world and decided to log out in the air. When I rejoined, the happy ghast was gone, and my character fell out of the sky to their death. Camels and boats have the same problem.
It appears that in the brief timeframe between the host disconnecting and the server closing, the game makes the remaining passenger the new controller of the mount. While this is generally expected in other multiplayer scenarios, it isn't when the controller is the LAN host, as the new controller will not be able to continue driving the mount anyway.
Steps to reproduce
Open a world to LAN.
Have another player join the LAN world.
Mount any multiple-player mob (e.g., happy ghast, boat, or camel) as its controller.
Have the other player mount the same multiple-player mob as a secondary passenger.
Disconnect, kicking the other player.
Rejoin the world.
Observed behavior
The mount will have become bound to the guest, meaning it's gone for the host and owner of the world.
Expected behavior
The LAN host would maintain control of the mount because they will always be the first to enter the world.
Same,and the mounts will appear when the player log backs. I use carpet mod and the mount will appear without passenger when i spawn a fake player.