On load some items gain empty nbt tags that prevent them from stacking with other items. I suspect this can also happen in other ways but cannot confirm exactly what happened with previous instances of this bug; I noticed some separated full stacks of items in my me system, gold ingots being one of them which would not drop from a block.

I tested this by mining end stone, it seems to only happen when the oddities section of quark settings is enabled (the jar itself being loaded does not cause the bug).

The bug can best be detected with nbtexplorer look for an entry on an item saying tags: 0, normal items have no such entry.

versions:
forge 1.20.1-47.1.3
zeta 1.20.1-1.0-16
quark 1.20.1-4.0-441
oddities 1.20.1
I am playing with a bunch of other mods but disabling oddities fixed it so they probably are not the cause.

I had the wrong mod (it was cold sweat), sorry for the irrelevant issue