Right now, all the wither storm does when you go through a nether portal is wait until you leave that portal. It makes more sense for the wither storm to send waves of withered mobs through until you go back to the overworld.

I am actually experimenting with making fleeing mobs go into portals, but this specific feature has been suggested #343

of course my specific search terms dont match #343

I am actually experimenting with making fleeing mobs go into portals

I'm interested to see how that develops though.