When landing an elytra at high speeds (say, after using a few fireworks), momentum still carries over. After landing completely, activating the elytra by jumping will launch you at a high velocity the direction you are facing.
It is a little hard to explain, so I have attached a video of the issue.
Ideally, after you land at a high speed, all momentum should be lost, and you shouldn't be launched when just jumping.
In the video attached, this bug appears twice, each time I land.
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