The bug

If there is no end island under the world origin in the end, the end exit portal generates at the very bottom of the world, just above the void. However, it doesn't generate completely; only the pillar in the center of the portal is there. As such, the player is unable to exit the end without dying and is unable to resummon the dragon.

A seed where this naturally happens is the following:

-1059792394137977866

Here, the main end island generates in a donut-like fashion, so that in the center of the island where the portal would have generated, there is a big hole, and the remnants of the portal are at the very bottom of the world just above the void.

Screenshots

screenshot of the weird end portal remnants

overview screenshot of the end