On spigot server (for more testing), i created a portal at 1024, 42, 1024, used it and adjusted position in nether to comply to 1:8 rule. Then i created a portal at -2, 42, 1024 (even more then 1024 blocks away), but i came out at my old, adjusted portal in nether instead creating a new one (creating a portal at ~ -200, 42, 1024 worked).

Vanilla server:
Portal 1a (overworld-old): 420,45, 1797(...1800)
Portal 1b (nether-old): 51, 45, 222 (...225)

Portal 2a (overworld-new): -618, 55, 1600 (...1603)
Portal 2b (nether-new) expected generation at: -77, 55, 200

X of Portal 2a once were at -610 but since it didn't work (even though more than 1024 blocks away), i recalculated related to x of Portal 1b, which was < 128 blocks, so i increased x to -618. Now distances of 1024 and 128 are given, but Portal 2b isn't generated, instead im coming out of Portal 1b.

I cannot tell why this is happening. I don't know which distance is more relevant, overworlds 1024 or nethers 128 radius?
As i understand, Minecraft is searching 128 blocks area in nether for active portals.
Is this 128 blockradius (respectively 1024 for overworld distance calculations) "increasing", when Portal 2b would generate somewhere beside the calculated spot (because of lava, other blocks), but then being closer than 128 blocks and then finding Portal 1b, which situation would prevent a new portal generation?
Is the game searching an active Portal around the calculated spot or around the closest possible spot (where a portal can be generated in nether)?

When i build Portal 2b manually, traveling works as expected.