The bug

If you use a flight duration firework rocket and land before the flight boost ends, if you activate your elytra again, the "boost" will still be active and you will fly as if you used a firework rocket when you have not. I've tried it with flight duration 3 rockets, unsure about the rest.

To reproduce

1. Equip elytra and fill your inventory with rockets
2. Switch into Survival mode
3. Deploy your elytra
4. Use a rocket to propell your flight
5. Immediately land on the ground
6. Deploy your elytra again
   → You are boosted without using rockets