When using fireworks during Elytra flight, fireworks will fly upwards rather than providing a boost.
Worldsave attached, activate Elytra and attempt to use rockets to boost.
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Very likely a duplicate of MC-111480.
If that ticket doesn't describe your issue, please comment here.

This is a duplicate of MC-111480. I was able to use elytra in your world without problems.
Did your fireworks have the
Flight
tag?