After I defeated the ender dragon in my survival world, no end gateway appeared. When I created new worlds using the same seed and defeated the ender dragon, the gateway did generate at about (-77, x, -56), but on my survival world it was not there. I duplicated the survival world by creating and unzipping a backup, and respawned the ender dragon in creative. After defeating it the second time, a new gateway still didn't spawn.

I've looked all around the island, and there is no gateway. Is there any way to get it to generate? I've tried deleting the region file of where the gateway should have generated (r.-1.-1.mca), but it didn't work.

I've attached the level.dat file of my world, as well as some screenshots of the area where the gateway should have generated.

seed: 706026619949723859