I was messing around with my friend in a world in the end. We made it so that when we died a portal spawned at us so we would instantly be teleported to the end again. I accidentally left before I could respawn so I rejoined. When I did it made it so myself on the client side could move, but my player on the server side couldn't. My friend said I was inside the building we built (spawn, we made it so the spawn, in the end, was in the building), but on my screen, I was above, as I had flown above it after rejoined.

This is just an issue and can't be used for any means to exploit