I have a farm the use nether portals to remove mobs from the mob cap so more can spawn. I linked all of the overworld portals to an singular portal in the nether. (-1033,164,-860 centre of overworld portals to -129,66,-107 in the nether) they all stay linked until a make a new nether portal in the overworld. it does not mater if it is 50 blocks away or 100 blocks in the overworld.
e.g. portal made at -979.161,-1050 overworld and -122,80,-131 in the nether, all the portals around -1033,164,-860 link to the portal -122,80,-131 when there is a portal that is already at it proper nether coordinates. I don't have this problem with a guardian farm that users the same principles. The guardian farm centre 3962,63,-2889overworld and in the nether 498,62,-364. the player portal 4171,64,-2850 overworld 520,61,-358 in the nether. both farm use the same printable but the new one unlinks every time when an player portal is added.