What I expected to happen was...:
When going through a portal from the overworld to the nether, it should connect only to existing portals within 128 blocks of horizontal distance from the corresponding coordinates.
What actually happened was...:
Overworld portals connect to nether portals up to 143 blocks away.
Steps to Reproduce:
Place a portal block at (0,0) in the nether
In the overworld place a portal at (1031, 0). This connects to the existing portal as expected, because 1031/8 = 128.75, rounded down it's withing the range.
Place a portal at (1032, 0). This one shouldn't connect, but it does. Same thing happens up to (1151, 0)
Place a portal at (1152, 0). This is the first one that doesn't connect
I tested this in 1.12-1.14 and the range is 128 as expected. The bug seems to appear in 1.15+
I haven't tested it going through the portal the other way, but presumably the same thing happens.
Comments 2
This has been fixed at some point, see MC-197538
I just had this with two portals approx 300 blocks apart (about +100 and -200 on the x axis) in the overlord. Second portal (I repaired an abandoned one) teleported me to the location of the first one in the nether, rather than creating a new one. And going back in returned me to my original/first portal. (latest bedrock, dedicated server)