I've seen while playing campaign after picking up grenades here and there from dead AI that even though I had gathered the same grenade types, they did not combine even in the same pack slot ( chest, fighter rig, or backpack). So I would end up with a 4-pack and a 6-pack grenades in the same pack slot or something along those lines.
- Legacy ID
Pick up grenades from different soldiers and locations and try to place them in the same pack slot. The engine may or may not think they are different grenade types.
It doesn't happen with every grenade. Some will combine, others won't and I'm not sure why the engine thinks that some are different grenade types.