The bug

In the end dimension (obviously) End Gateways spawn on the sky islands. However, I found a portal at 1495, -4108. When I went thru it, it teleported me to a portal at 319 -956. Then I walked across the island and spotted a portal at 362, -1166. I went thru this portal and it created a new portal at  319 -955, shifted one block to the west and up about 8 blocks.

How to reproduce

I am not sure how to reproduce. I have only generated one End Gateway on the original island. This one is positioned at -77 75 -56. I travelled bridging across islands so I might have come across portals that are not working yet.