Rockets are not deploying while in flight mode. This was not the case for some time. But now it is. The player can still deploy rockets while on the ground.

Steps to reproduce:

1. Get an elytra and fly. While in flight mode, use a rocket(like a lot). (Point to be noted: rocket works the first couple of times) 
2. Soon, the rockets become undeployable. 
3. Repeat the above steps for the effect you are looking for. 

OR

1. Get an elytra and a chestplate. 
2. Equip the chestplate. 
3. Take the elytra in the mainhand (off hand is not possible, don't try that) 
4. Jump off a cliff or building or a high place. 
5. While in air, equip the elytra and use the rockets alot.
6. Repeat the steps 1-5 until the necessary/unnecessary results are made. 

I am not sure which will trigger the bug.