When landing with the elytra, apparently there's a chance you enter this weird state where your character looks like it's still flying, but it's walking at the same time, and when you jump you fall as if you're using an elytra, and you can't use fireworks. I couldn't get back to the normal standing position till I re-logged.

I recorded a (low quality) 1 minute clip of me walking around in that state:

https://youtu.be/ctzcnpp5Vfc