During the End battle I was expecting that, after I killed the Ender Dragon, The empty End Portal would activate.

When I killed the EnderDragon I was flying right above the empty portal where I knew I would not fall in when it was activated (I was in Creative). Right as I killed him, The End Poem began. So I went back to The End to see what happened. Another End Portal, this one active, had spawned a few blocks over the inactive portal (thus why I had been sucked in).
The attached image shows exactly what I am trying to convey.

Not sure how many of these actually matter to create the bug but I might as well be thorough...
1. Create an End Portal in the Overworld in Creative and go to the End.
2. Almost kill the Dragon. I used a combination of snowballs, poison arrows, spectre arrow, harming arrows, and a diamond sword.
3. Kill the EnderDragon while flying just above the portal with a diamond sword.