I have had the same problem but it also happens when you have 4 empty inventory slots which the drum mag could fit in to.

I had 4 empty inventory slots (2x2). I went to drink and pressed reload by mistake. The drum mag didn't go into the empty inventory slots but appeared on the floor besides me (so didn't totally disappear like it does when you have no free inventory slots).
This was also when reloading to a 30 round AKM mag.