In my world I had only ever fought the dragon twice to get the advancements, but recently I decided I wanted to build in the end and my plan was to spawn the rest of the end gateways, so went to go spawn the ender dragon 4 more times with the 16 end crystals I had. but the first 2 fights didn't spawn a gateway. I presume it has something to do with me being on a different version compared to when I fought it the last 2 times. I don't remember what version I fought the first 2 dragons on but it was either 1.16.x or 1.17.x