If you place multiple End Crystals around the End Gateway, they are teleported too and stucked together. It looks really like a bug.
There should be an exception for End Crystals to not to be teleported through the portal. From POI of the player it's a just a block, like sand or dirt. No matter what's the technical background.
It was most probably v1.9.2.
Maybe (or most probably) it isn't a bug, because someone has spawned probably the dragon with NoAi:1 previously.
I didn't know that this is possible, so I thought this was a bug. I'm sorry...