Nether portals exiting into a different location

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.

Any suggestions?

The coordinates aren't lining up. You should rebuild your nether-side portal 11 blocks further north at z=-6.

I've had the same issue with one portal that took me into the nether, but the way back brought me to a slightly different location; roughly 50 blocks away from the original portal.

I destroyed the second overworld portal, took off all my gear except for a single pickaxe, went into the nether, destroyed the nether portal there, then jumped into the lava to die and respawn in the overworld. That fixed the problem for me, as the newly created portal worked fine after that.

I'm still not sure what causes it, but my guess is something like a rounding error, where the calculation whether any existing portals are in range, is thrown off, causing the game to spawn a new one.