I'd be surprised if you cant reproduce this. Basically in vanilla nether portal mechanics when an entity is sent through a portal it is given a 15 second portal cool down where it cannot go back through. This appears to not be the case with interdimensional portals, since these mobs are flickering, I believe it means that they are going back and forth between the overworld and the custom dimension repeatedly every tick. This probably isnt great for lag and is surely unintended.