Not 100% sure if this is a bug, but I cannot seem to resupply half used magazines/ammo boxes from the resupply boxes (tapping f). Holding f to resupply the whole loadout solves this issue.
Only tested a few guns in the shooting range and have experienced in co-op
If you run an AR or any box mag fed guns, this isn't a big deal, it's easy to discard mags or have enough in the first place. The issue comes when you run any belt-fed MGs and have belts with low rounds and cannot quickly resupply.
So yeah, not too sure what's going on and haven't seen anything regarding this posted by anyone. Briefly checked the change logs, haven't seen anything regarding a change to this either.
If anything, might have something to do with the recent patch and grenade launcher reloads? (total guesswork)