The mobs when they cross the nether portal to the overworld get disappear or teletransported again to the nether, the only way to do for the mobs can teletransported to the overworld is cross the nether portal 2 times.

The mob that it happens more often is the allay, they always get teletransported and this is pretty annoying becouse they dead in the nether or they get lost.

That may happens with any mob in the game, I recorded it with the frogs and the allays