I know that this isn't an issue on my part, since I have tested this on multiple computers. Whenever you deploy the Elytra, as soon as you hit the ground use a rocket. This will cause the rocket to shoot you up, but since you hit the ground, the Elytra doesn't let you fly. The result looks like a large jump. This is repeatable, and not a one time thing. This was tested on 1.17.1 and the latest 1.18 snapshot, 21w38a, and I have a video to show it happening in both versions.

An explanation on why the bug works: When you fire a rocket, it has to do a small startup animation first. After the startup, you get the momentum given to you. This bug is when you land with the startup animation playing. The momentum is always given at the end of the animation, so when you land the momentum is given, but the elytra isn't active since you landed. I hope this explanation will suffice.