whenever I'm still flying with elytra and the portal teleports me, it will move me far out, sometimes with a portal and sometimes without. This is very similar to the one where you would eat in a portal and be teleported a distance. It works going to the over-world and to the nether. I have been able to replicate this in creative and survival, also is not dependent on the specific world. here is a link to a video of the glitch https://youtu.be/9xTjG02zly0