I have lost several pistol magazines for the P1 and Makarov while attempting to reload the magazine or using the "R" key to switch magazines.
In the first case, I had a full inventory, and needed to reload a partially empty magazine, so I removed the magazine from the pistol (Makarov) and placed the magazine on the ground to reload it; the magazine disappeared and could not be found after searching carefully. This happened twice, so I vowed never to place a magazine on the ground again. Currently, if my inventory is full, I place ammo on the ground to create an open inventory space, remove the magazine to inventory, and reload it from the ammo on the ground.
In the second case, I was travelling with full inventory, and my P1 pistol was inside a protector case with two additional full magazines, and I access the pistol via hot key. While running for my life and trying to reload, I pressed "R" to try to swap magazines. A new magazine was loaded, but the empty one was not placed back in the inventory, and disappeared (again, this happened twice in a row) I searched the ground on the area to be sure, but could not find any empty mags.