I've recently upgraded my 1.19.4 world to 1.20.1. I've already got 2 existing gateways in the end. After updating my world, I killed ender dragon once again but it didn't generate a new gateway. The game kinda re-regenerate my existing gateway and it arrives the same point.

NOTE:

This happened twice but in the first time I was using iris shaders and the issue flagged as invalid. This time I've replicate the same steps with plain vanilla Minecraft, no mods.

To duplicate the issue, you should have a world played in 1.19.4 and has at least one gateway generated in the end after beating ender dragon. Create a platform around the gateway so you can see the disappearing blocks when the bug happens. After close the game and update your world to 1.20.1 with minecraft launcher. Then re-spawn and kill the ender dragon to generate another gateway.