One thing I noticed with this is that it only happens to my “female” Night Elf Hunter. My Night Elf Druid who is “male” never has any issues. I did a few racial changes a few months back on a priest who was a NE and “she” also would drop through the world after entering the portal.
I have recently got to the point of just flying over to Darkshore and go back in time to the Cata version Darkshore before I leave. I don’t drop through the world then from Boralus going to the Cata Darkshore