I built a fort in my creative world, and near it an Ender Portal, to see if the End is flat in an all flat world. I traveled to the End, I killed the Dragon and returned back. I then destroyed the portal blocks and the eyes, but the interior (the black part of the portal, that looks like Space) remained. I can destroy it, but I can't take it. You can also put blocks on it, but not move it with a piston. I tried building another portal, but it happened again.
It doesn't work with Nether Portals.
I don't have mods, texture packs or resource packs.