Hi, I made a 1.12 modpack and have been alpha-testing it (it's not uploaded yet) for about a month.
My constant problem is that items no longer stack. The second stack of items always has an empty NBT tag, but why? For example: ingots, redstone dust, bones, seeds, wheat, gunpowder, zombie flesh, bread, apples, TechGuns heavy cloth.
I notice it when looting chests, or when picking up mob drops. Also when I shift-click items from my inventory into a chest (or the other way round), they arrive as two separate stacks. I click one of them. One second later they appear as one stack. I close the UI. Seconds later I notice that some items have returned to my inventory! >:-( My fun raids have turned into a frustrating exercise of inventory management... -___-
Dropping the two stacks and picking them up does not fix it. Crafting e.g. redstone dust, or ingots, into a block and back fixes these cases, I think?