End reset (delete file DIM1): no dragon (stuck in the End)

In my Vanilla multiplayer server, I resetted the End.
First time going through the stronghold portal I fell into the void, my friend did spawn in the end. He saw the dragon for a moment before it disappeared forever.
Second time I deleted the End folder (DIM1) we both spawned good in the end but no dragon was to be found.
Tried updating to Pre2 and resetting the end again. Still no dragon.