A few days ago I created a new survival world on the 20w20b snapshot. On May 20 snapshot 20w21a was released and I updated my survival world to the latest snapshot. On the same day I defeated the ender dragon but the End Gateway portals, which normally appears after defeating the dragon, did not appear.

Now, I have tried beating the Ender Dragon in a different world on the same seed in creative mode and the End Gateway did appear. I even did it twice, both times the End Gateway appeared.

On my survival world I have killed the Ender Dragon three times but the End Gateway portal never appeared. Before the third attempt I selected the 'optimize world' option in the select world menu, but still no Gateway portal.