Spawn Under Nether Portal after traveling to the overworld

I have a skybase in my SMP world. It is built out of slab floors and has a nether portal on it. 75 percent of the time I go through the portal, I spawn a few blocks under it and fall to my death.

https://youtu.be/3yAkf4n5Rbs
note: the last part of the video with gameplay is slowed down.