Sometimes Boats and Animals start glitching in Multiplayer which causes them to turn invisible. I dont know how to achieve this but its probably because of bad connection.
How to reproduce:
Open a world.
Ask your friends to join.
Go as far as you can from your Wifi till the Connection is not stable.
Tame an Animal or just ride a boat.
Ask your friend if they can see Boats or tamed Animals.
Expectation:
Normal Gameplay with Entities everyone can see and interact with. No vanishing cat boat or wolf etc.
Reality:
Cant see Entities and dont know where they are. Do they even exist?
Your steps to reproduce include deliberately making your wifi connection unstable. Therefore, you are not reproducing a bug in the game. It's not a bug that other players can't see a mob you spawned when your device is unable to transfer the mob data because of an unstable wifi connection.