I'm not quite sure if this is a bug or just a change in gameplay.
In older versions Mobs used to be able to travel through nether portals.
Since 1.9 this does not work anymore.
(They disappear on one side, but wont materialize on the other)
Since 1.10 Mobs that have been spawned using an egg may again travel to their natural habitat, but not the other way.
(a blaze may travel from overworld to nether, but not from nether to overworld)

So far i tested Skeletons, Wither Skeletons, Blaze, and Magma Cubes.
I hope this is not permanent or else this is a real bummer for my mob zoo.
cheers