I've been having issues with a Nether portal - when I build the portal and enter the Nether, everything is fine. The portal is in the right location, I've checked the overworld coordinates, divided them by 8, and they match up to the coordinates in the Nether. Except when I exit the Nether, it takes me to a new portal a distance away. I can use that portal, but it won't let me use the original one - when I try to disable the new portal and use the original, it puts me somewhere different instead. I've had this issue both on the Mushroom Island where I live and another smaller grass island - on the grass island, both attempts of exiting the Nether put me on a platform over the ocean.