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:

1. Place a portal block at (0,0) in the nether
2. 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.
3. Place a portal at (1032, 0). This one shouldn't connect, but it does. Same thing happens up to (1151, 0)
4. 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.