have you checked the console for overload messages?
So sorry for the late reply! We didn't get a chance to play for a few days.
I went in the nether with my friend twice (so I could /tp him back if needed). Both times it didn't teleport him, but the first time I couldn't see him and any items he picked up appeared like they were coming to my player. The only time there was an overload message in the log was immediately after I entered the nether, and it didn't teleport me anywhere. I'm hoping the problem just decided to go away and won't occur in the future.
Thanks for your suggestion ![]()