I've created a number of railroad circuits inside a building, started minecarts circulating on the railroads, and had the minecarts coincide with spawned animals (I've tried and verified this with pigs and cows) such that the animals ride in the card. This all works fine. However, when I leave the building, walk a distance away, and return, all of the animals are hovering in the air at some specific location (possibly where they were when I left view of them?). The minecarts are still moving around the railroad circuits, and the game clearly thinks the minecarts still contain the animals, because you can't get in the minecart. If you destroy the minecart, the animal disappears from where it's hovering in the air and appears walking away from where you destroyed the minecart. You cannot affect the hovering versions of the animals in any way. Destroying the minecart then killing the animal, then re-creating both and getting the animal to get back in the minecart, exiting and re-entering causes the same problem again.

I performed this on a Realm, and had a user using a PC log in as well, and they observed the same behaviour, so it doesn't appear to be my specific computer/client.