Nether Portals don't spawn "return" portals when you go through them

I'm running a server on 1.16.1

I'm having an issue where sometimes (not most of the time, but every so often) a player will enter a nether portal and come out the other side and there's no "return" portal spawned in the nether to get them back.

Maybe the portal is spawning away from where they are, but that's not clear to me. Either way, regardless of whether or not you build a "return" portal in the place of the missing portal, you are ALWAYS returned to the exact same spawnpoint in the Nether when entering from the overworld.

This could allow players to build blocks around that spawnpoint and suffocate players entering from other side.

I saw a similar issue in the Bedrock project so I thought I'd post here.