IRL throwing away empty or near-empty mags is one of the dumbest things you can do. Magazines don't grow on trees and are nearly always re-used, also in WWII. And that one round that's left in one of your magazines? It can save your life.
This sums up the thread. Yes, occasionally you do drop the magazines. If you absolutely needed to reload very quickly you might drop the now empty magazine to reload quicker, but implementing this would be very clunky in a game.
So we should default to storing the near empty or empty magazines.
That being said, it looks like the player discards the drum when reloading the PPSH in some of the RO2 videos...