Steps to Reproduce:

1. Grab paper and gunpowder from the Creative Menu
2. split the paper and gunpower into 2 halves
3. Craft rockets with one of the halves, and then craft the other half.

Expected Result:

The firework rockets should stack if they have the same flight duration

Observed Result:

The firework rockets of the same duration don't stack

How to fix:

1. Grab those different crafted firework rockets of the same flight duration in different stacks.
2. Open the inventory and left click one of the individual stacks.
3. Exit out of the inventory, and the fireworks will stack like they should.