Steps to reproduce:

1. Get a mob in a minecart. I've tested with villagers, skeletons and zombified piglins.
2. Drop the mob on a nether portal by passing over a activator rail.
3. On the exit side of the portal, have an empty minecart on rails right next to it, like waiting to catch the mob as soon as it decides to move away from the portal.
4. Follow the mob through the portal. On the exit side, the mob should be already on the once empty minecart. If it is not, try a few more times and maybe change the orientation of the setup to be able to reproduce the issue.
5. Move the minecart a few blocks away from the portal and get the mob out of it, either by breaking the cart or passing over an activator rail.

Observed Result
The mob will teleport. In a survival world where I noticed the issue, villagers were teleporting back to the dimension they came from, to a place close to the portal entrance. In a creative flat world where I was trying to reproduce the issue, the mobs were teleporting right next to the exit portal, where the minecart that grabed then was before.