This has happened to me twice while trying to open a door. The first time, I thought I had dropped the 75 round drum and went looking around the entire building for it, without success.
EDIT: I tried to reproduce the error described earlier without success. The actual bug is that the 75 round drum disappears completely when reloading. I tested this on open terrain the drum really is gone after reload.
Also, please can you change the settings so that reload-mechanics are no longer triggered with the same key as door/item interaction, since this is very annoying.
There is a key intended solely for reload that can be used for anything to do with switching mags or snaploaders, this does not have to be the same key as opening doors.