Items like fireworks and banners, which can have NBT and stack, cannot stack while they have NBT.
To reproduce, try crafting some fireworks or applying patterns to banners then attempting to stack them (shift-click or otherwise).
MC-129282
Items with NBT cannot stack
Resolved
Fixed
3
Cannot reproduce in 18w19b, it only happens if the nbt isn't an exact match, for instance applying the same banner pattern in a different order.