On the 10 person realm I own, when entering a nether portal I’ve already built, occasionally the game will place me nowhere near a portal in the opposite realm, at the same coordinates I entered. For example, I would enter a portal in the overworld at x=800, z=64, that already has an equivalent portal at x=100, z=8 in the nether, but instead of ending up in that portal, I end up at x=800,z=64 in the nether, nowhere near a portal. So far I’ve had this occur three times. The second time was by far the worst, sending me to roughly z=12,500 in the nether with no way back, instead of sending me to the portal at z=1,500 that it already connected to. These occurrences are random, and none have been in the same portal.

It occurred to me going both ways. Once going from the overworld to the nether, and twice going from the nether to the overworld.