Pets can be teleported to the incorrect coordinates when going through a nether portal

On certain seeds and with certain coordinates, when a tamed pet travels through a nether portal, it can teleport to the owner’s overworld coordinates in the nether (instead of being on the other side of the nether portal) and vice versa when traveling from the nether to the overworld. It seems to be a chance for the pets to teleport to the wrong coordinates, but some coordinates I have never had it happen and other coordinates it happens nearly every time. We have not figured out why only some coordinates and only some seeds work, but I have attached a list of coordinates we tested (all for 1.21.10 - I only tested one but confirmed it still works in 25w43a). Not a single negative x negative z coordinate portal incorrectly transported tamed pets, but this could be small sample size.

I have been testing with tamed parrots, but tamed wolves and cats are also affected. Untamed pets and sitting pets do not attempt to teleport, so the bug does not happen. In one instance, I put two birds at the same time through the portal and had one teleport to the wrong coordinates but the other was fine.

In addition to some coordinates seemingly never transporting the pets to the wrong location, some portals transport parrots into the nether without a problem, but when moving them back into the overworld, the bug happens. Some portals can transport parrots without issue from the nether to the overworld, but fail when pets go from the overworld to the nether.

See attached video for one of the trials I performed, I had to lower the video quality in order to attach it to the bug report.

How to replicate:

1. Start a normal world with this seed: 5130415253217361179

2. Teleport to 1002 64 976 in the overworld and build a nether portal.

3. Spawn and tame a handful of parrots.

4. Bring them through the portal, I found this to be easiest in survival (see video for my method).

Observed Result:

Some birds may be on the other side of the portal, but I’ve found that at these coordinates the majority of birds teleport to the wrong coordinates. Change to spectator mode and teleport to ~1002 66 976 (the place you were standing in the overworld) in the nether. You will find the missing birds here.

Expected Result:

All birds would be at the nether portal on the nether side.