[2:02 AM] tduva: hmmm
[2:03 AM] tduva: I recreated Josh's nether portals
[2:12 AM] tduva: He build his Portal at (-136, 17, -74), which translates to the Nether coordinates of about (-17, 17, -9), at that place in the Nether there wasn't any cavern, so it put it at about (1, 16, -8), those coordinates translate into the Overworld coordinates of about (8, 16, -64), where it then starts searching for an existing Portal in a 128 blocks bounding box horizontally. The original Portal is 144 blocks away in the X direction (-136 -> 8), so too far for the game to choose it, so a new one was created at (1, 21, -55).(edited)
[2:14 AM] tduva: I already thought that it must have been something like that (the game creating the Portal due to obstruction in a location where it then can't find the original on the return trip), but I didn't realize immediately that by the game moving the Portal only a little bit in the Nether, it could easily move further than the 128 bounding box it searched in the Overworld, because the bounding box that it searches doesn't scale as well between the dimensions, it seems. So it doesn't have to move it more than 128 blocks in the Nether due to obstruction (just more than 16 or something), for it to then be too far away in the Overworld.(edited)
[2:14 AM] tduva: It's easy, but also confusing at the same time. :open_mouth:
[2:53 AM] tduva: Oh, I only just saw that he made another one, but I assume it's the same issue.
[3:01 AM] tduva: So the second Portal at (-414, 66, -256) translates to  (-51, 66, -32), which seems within a 128 bound box to the already created Portal at (1, 16, -8), so it send him there. But the older one that was created when travelling back into the Overworld for the first time was of course created much closer to that, so the game chose that as an exit again.
[3:02 AM] tduva: Aren't Nether Portals fun. joshHorse
[3:15 AM] tduva: Not sure why it created the new Portal above ground though, after he destroyed the one he always exited through.
[3:16 AM] tduva: It did when I tested it as well, but at a slightly different position, might have to do with how I went throught it, not sure.
[3:28 AM] tduva: And the next Nether Portal he got was the same I got, but the one after that completely different for me.(edited)
[3:28 AM] tduva: Nether Portals. :joshThinking: