If I push a mob, say a villager or a horse, through a nether portal, sometimes the mob will not appear on the other side.

I believe that the game is trying to spawn the mob on the other side but after a single attempt if it can't find a valid space then it stop trying. I believe this because the problem is worse for larger mobs (horses) and better if portals are larger.

This bug has occurred for both me and my friend playing on PC, Xbox One S, and while playing on realms.