I have been working on producing fireworks that announce the start of nighttime. I had started producing fireworks with duration 3, white big balls. I had produced two batches; one of 19 and one of 8 (blame inconsistant gunpowder supplies). I had previously launched one from the batch of 19, as sort of a test fire, before going out for more gunpowder. After coming back, I produced the batch of 8, and tried to merge them with the existing stack of 18; the two stacks swapped instead, as they do when trying to swap different items. Eventually, after fiddling around with splitting the old stack, I was able to merge them all to a stack of 26.

I had found possible duplicates in a search prior to creating the issue; however, they were all closed as incomplete. Note however that I had only been looking at the summary field, on a search of 'MC firework stack'.

Possibly related (inversely) to MC-5214. That issue was resolved as fixed; the "fix" might be causing this bug. However, it being fixed was merely an assumption.

If I get the issue to occur again, I'll hold the stacks and upload my level.dat file. I still have to craft 6 more to get my desired count of 32 fireworks; I'll craft them in groups of 2.

— Receipe Trace —
6 BlazeRod -> 12 BlazePowder
11 Coal + 11 BlazePowder + 11 Gunpowder -> 33 FireCharge
11 Bone -> 33 BoneMeal
33 FireCharge + 33 BoneMeal + 33 Gunpowder -> 33 FireworkStar (White, BigBall)

57 Gunpowder + 19 Paper + 19 FireworkStar => Fireworks[19]
Fireworks[19] + RMB => Fireworks[18]
– Save? –
24 Gunpowder + 8 Paper + 8 FireworkStar => Fireworks[8]
Fireworks[8] + Fireworks[18] => Fireworks[18] + Fireworks[8]
– Messed around with splitting and merging stacks –
=> Fireworks[26]
– Save –
18 Gunpowder + 6 Paper + 6 FireworkStar => Fireworks[6]
– Does not merge with saved stack –
-- Save –
-- Upload level.dat –
Fireworks[6] + Fireworks[26] => Fireworks[32]