NOTE: this only happens when you have a simulation distance of 6
NOTE2: attached is a copy of a world where it happens, go to /tp @s -344 -17 1100{}

Nether portals created below y=0, when entered, and then exited through the nether, do not link back to the portal where they were created, but back to a new portal that is above y=0

Steps to Reproduce
1. create a new world
2. go to below y=0
3. create nether portal
4. enter said portal
5. go back through from the nether side
6. notice you are at a different spot above y=0