Before updating the world to 1.16.1, I had killed the dragon twice in 1.15.2 and gotten a gateway portal each time. Today, I killed it two more times, in 1.16.1, but no portals spawned. This is on a server that I play on with one friend. I do not know if it is a multiplayer bug, if it is a bug with spigot, or something like that. I tried to replicate it in a vanilla singleplayer world but everything seemed to work perfectly normally.