Known Seed in which it can be replicated (haven't successfully tried others):
-1963576890
Overworld Portal coordinates:
281-284 (4 blocks wide), 61, -16
Replicate issue by:
Building a portal at the coordinates above (not far from spawn).
Light and enter portal.
Walk around, at least going down the soul soil/sand hill on which you enter (Nether Coordinates ~37, 49, 15
Climb back up the hill re-enter the portal from the Nether.
Player will return to Overworld at coordinates 305, 68, 17
One wonders if the sign is getting swapped somehow (bolded above for emphasis).
Demonstration video:
https://1drv.ms/v/s!Apr73Jy6zWsqskUmCBUy8Mi6jcjg?e=Zfnzv8
Comments 3
The overworld portal I created in the cave is/was my original. The nether world portal was generated by this portal. In a newly generated world (this seed), if I go right back into the nether world portal I come out in the cave. If I walk around the nether and enter the nether portal, it brings me out at the new portal.
Based on your comment, I think the bug might be that the game-generated portal in the nether is made too far from the overworld portal I made. So when I try to go back to the overworld the way I came, it does the math you talk about and determines it should make a new portal in the overworld.
It depends on the exact coordinates of the player when you enter from the nether side (the first time).
When I rewatched your video when you come into the nether you are at 38,50,13 this corresponds to overworld at 304,50,104. If you go back through the portal at those coordinates, this will be within 128 blocks of your Original portal, so it will take you to that one.
However, if you return from slightly different nether coordinates, when multiply X & Z by 8 you end up more than 128 blocks from your original portal, so the game creates a new one. After that, the newer overworld portal will always be closer so it will take you there.
In general, portals are not specifically "linked" in that the game does not "remember" which portal you came through, but instead will calculate the closest portal every time. If you want exact control, after you get to the nether create a new portal at coordinates you calculate based on the overworld one and then break your old portal (or break it first if you like).
The video shows the coordinate as:
Original: 282 61 -16
Nether: 38 50 14
Return: 305 68 117
The calculated Overworld coordinates corresponding to your Nether coordinates are 304 50 112. The distance from there to your Original portal is 135 blocks. This is more than the 128 block search radius, so a new portal is created. For more information on portal linking there are many tutorials online and the minecraft wiki may also be helpful.