The items dropped by the mobs have empty NBT tags and this prevents them from stacking from normal items. Examples are coal and iron ingots.

pWn3d1337/Techguns#122

Submit bug:
Reaction Chamber make Anti Gravity Core only causes explosions.Has been tested many times.

Please forgive me for my bad English.

I have the issue too
and its still occurring on the latest version (1.12.2)
it mostly come from zombie miners if I can recall.

The newest test releases have this fixed: https://github.com/pWn3d1337/Techguns2/releases