This has just started happening recently. This bug happens only on LAN worlds too, but not for all players. It only affects the host of the world for some reason. Even stranger, the player connecting to a LAN world hosted on the same computer does not encounter the bug. Their elytra works just fine with the fireworks.

Usually when flying using the elytra, you can use a firework to boost forward very fast. This doesn't happen with this bug. When I use a firework while flying, it just shoots straight up as if I used it on a block and it doesn't boost me. I just stay the same velocity as if I never used it.

This is a demonstration: https://youtu.be/RcZMBTnTdCg