When using a firework rocket whilst flying an Elytra, doesn't give you a boost but instead spawns a firework rocket like you use it normally. Note that this isn't always easy to reproduce because it doesn't always happen. When using the same world in the same version, after relogging this bug occurs.

Steps to reproduce:
1. Equip an Elytra and get some firework rockets.
2. Start gliding with the Elytra.
3. Whilst gliding, use a firework rocket.