Due to the lag created by elytra loading lots and lots of chunks (I'm sure this has been reported dozens of times), fireworks are able to propel elytra for much longer than should be possible. I went from y=400 to y=1300 with one firework. This allowed me to glide over 10,000 blocks without even moving my mouse.
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Does MC-90026 describe your issue?
A lot of things behave weirdly when there is server lag, not just fireworks.
To fix this they would need to fix the lag itself which is the cause of the problem.