The bug:

No matter where I build a nether portal in the overworld it links to the original portal in the nether.

it seems that its unable to generate new portals in the nether and thus defaulting to the only one there.

Details:

This is on my friends realm (i can easily get him to supply any details if you need). the first portal in the overworld was built at pos OW(-125, 72, 4), this takes us to a portal in the nether at pos N(-26, 73, 8)

now since its a realm, and i did not make the original portal, i dont want to break or mess with them, nor build on that floating platform a couple hundred blocks above lava. so i made a minecart track (in the overworld) away from my base to make my own portal where it would not interfere with the ones mentioned before. i read the wiki article about nether portal POI search and portal generation etc and worked out where it needed to be. when i built this portal and entered it it took me to the original portal in the nether N(-26, 73, 8), and going back through to the first one built OW(-125, 72, 4).

i thought i must've done the math wrong so i did another even further away but the same thing happened. since i had already used 3 stacks of iron and a stack of gold to make this track there, and something clearly wasn't working properly, i stopped making the track and just went even further. 

I build a portal at OW(339, Y, -531) which, when entering the portal takes me to the original portal in the nether at N(-26, 73, 8), and therefore exiting at OW(-125, 72, 4)

edit to clarify: before i built the further away portals i took down the previous one i had built so there was at most 2 active portals in the overworld at any time.

the N(-26, 73, 8) portal is clearly not within the search region of the OW(339, Y, -531) portal, so the portal creation (in the nether) must be failing and thus defaulting to the only available portal.

(for what its worth, i also noticed (on this realm) some odd behaviors with villagers linking to workstations that i hadn't seen on bedrock before, but i doubt this is relevant for the bug mentioned above)