This only happens when firework rockets are generated via command with no NBT data other than a custom display or name (meaning no decoration). When the stack is split in some way and one of the smaller stacks is used, that second stack loses the ability to stack with the unused set. This includes use when launched, used with an elytra, and shot out of a dispenser. When an additional stack is used, it can combine with the other used stack again.

Further inspection with advanced tooltips shows that the number of NBT tags changes from 1 to 2, and an even closer look reveals that "Fireworks": {} is added to the NBT after a stack is used.

The attached video shows this in detail (poor quality to fit under the 10MB limit).