We going through a new portal built in the over world, it will link up to portals that are well over 500 blocks (overworld counting) away instead of generating a new portal properly in the nether. 

Overworld portal -742, 55, 305 sending me to a existing portal  in nether at -32, 45, -62, which then sends me to -289, 109, -559 in overworld. 

Seems like nether portals are not generating a portal on there own if there are ANY other portals present for it to link to? 

update: Tested in creative. Made a portal at 64,64,64 overworld. the game properly made a portal at 8,52,8.... Go back through portal to overworld. Then build another portal at 64,64,560 in overworld. Go through and it incorrectly links back to the portal at 8,52,8 instead of generating a new portal properly in the nether.

If manually build a portal in the proper nether location at 8,64,70, (eligible area to generate a portal fyi) then it will properly link up to the overworld portal at 64,64,560. NOW, if i then break the portal in the overworld at 64,64,560, go back round through the orther portals, the transistion from the manual nether portal at 8,64,70 to the overworld WILL properly generate a portal in the overworld at 60,4,572. 

Conclusion:This is a on-way bug when travelling to the nether, portals will not generate properly if there are ANY other portals in the nether it could link to.

update: tested it from 64,64,1040 overworld... still links back to the 8,52,8 in the nether.