Hello there, I dont know if this bug is known because I couldn't find anything about that, I'm sorry if there is.

It happend in Multiplayer and so far i noticed this issue for the first time. Had done this before without a Problem.

I wanted to get some animals from the jungle a few thousand blocks away to my base so I created a second netherportal to minimize the distance. I tried with mobs that can follow after taming so far, in this case a parrot and a dog and the only untameble was a lava strider but that didn't had a problem, maybe as he only exited the nether but didn't entered before. Both the dog and the four parrots went into nether fine. The leashes were dropped and the dog as well as the parrots teleported to the nether. After i got them to the other portal und pulled them through with the leash they should have exited into the normal world, instead they vanished and only the leashes were dropped again. After that i didn't even wanted to try with a panda or an ocelot in fear of their loss. That's kinda sad as it's quite a struggle now to transport animals from other biomes over far distances, Maybe that rips some farms as well, didn't wanna try with villagers either, as i would cry for this fuss for nothing if they'd vanish. Dunno if it affects them, maybe someone else does. Please solve that, would be much appreciated.