Randomly teleported when entering Nether?

I have a modded 1.12.2 server. About 3/4 of the times my friend goes through a nether portal (overworld > nether), he gets randomly teleported in the Nether up to thousands of blocks away from the portal.omegle discord xender When he goes through the other way (nether > overworld) his player appears in the portal on the overworld side and he immediately leaves the game. When he rejoins, the game says he suffocates in a wall but his items don't appear. This has never happened to me. Any suggestions would be appreciated.