When I tried to cross the end portal with a Shuker, I didn't find it in Overword. After a while your drop appeared in spawn. I did some testing with various mobs and found that after a while they fell from the sky and died from fall damage. A few more tests and I found that mobs appeared in the overword above layer 320, and dropped for 5-10 minutes to the ground.
attached I put prints of one of the tests with cats (because they didn't die with fall damage). realize they're coming from the sky above the edge of the world