This was a bug? Are you sure? I don't remember it in previous version.
For all I know, swapping is always 0 time with DG trait. So it doesn't count at all in calculations.
Also when you reload, you don't drop magazines. You just reload the remaining bullets which usually takes as a full reload depending on weapons.
Since we are probably talking about pistols, you reload both when at least one bullet is missing from each. This obviously takes same total time as reloading both separately. It's also obvious that most of the times you don't want to reload both guns near enemies. It'll take time so it's not advised. Therefore it's ofc a better option to just reload one of them, and kill off the enemy. Reloading both takes too much and might get you killed in such situations. Which is fair.
Also, reloading when one of the two guns is already full, will just reload the empty gun, with it's normal reload time afaik.
Personally I don't see anything wrong with any of the above, it's simple logic.
Unless I'm mistaken and there is indeed a bug I overlooked.