How to replicate:
1.Create superflat world.
2.Put the function(attachment) into your saves/Testing-Function1/data/functions/function(create this folder)
3.Load world and do /gamerure gameLoopFunction function:bombCraft
4.Put the advancement(attachment) into your saves/Testing-Function1/data/advancements/bomb(create this folder)/bombCraft(create this folder)
5.Load the world or do /reload
6.Do /scoreboard objectives add bombCraft dummy
7.Now put 1 fire charge in upper right corner of your inventory and 1-3 gunpowder 1 slot bellow it. This is how you create small/medium/big bomb. The problem is that it doesn't, or at least it doesn't do it visualy. Instead of the firecharge, a firework charge should appear with custom lore and name. If we "upgrade" it with redstone/tripwire hook(put in 1 slot bellow firecharge) we can see that it keeps track of which bomb tier it was, but visually the fire charge remains.
Put your questions in the comment. I suggest that this bug gets reviewed by someone with little commands/function/advancments knowledge since it may be hard to understand the code for someone with zero command knowledge.