I'm having the same issue one an older world I made back in 1.14.2. I killed the ender dragon like 5 times earlier today, but I only have 7 end gateways total, and my statistics say I've killed 14 dragons (which is actually off, I've killed more- the world corrupted twice so that may be a factor). I'm playing in 1.19.4 on the seed 1939338204 btw, not sure if that has an effect. Also important to note I'm using optifine and have used a mod before on the world that fixed some bugs with bee ai (I'm not currently using that though), but other than that it is vanilla.
Edit: I know for a fact it said 14 dragons earlier today but now after reloading it says 12. Weird
found in my 1.18.2 world two of these in a warm ocean. there was land nearby so its not like it couldnt spawn. all the other maps ive found have been find, just the two in the warm ocean. the world seed is -1740081556436845317 and the coords are 4293 -2001 for one of them. dont know the other coords rn but its nearby.
I spawned it using enderpearls as well.
I've updated my survival world to 1.20 and I'm still experiencing this issue- they just never spawn for me. I have 7 even though I've killed at least 15 dragons (don't know exact amount cuz my statistics are wrong)