Platform: Xbox One - Single Player
This seems to occur when picking up a sidearm that is NOT found inside of a container. Picking it up will cause your current sidearm to appear to be deleted, or that it has disappeared. Video below seems to indicate that it remains in your inventory, but it cannot be equipped. Notice that ammunition can be added to multiple sidearms of the same type - also that 9mm can be added to a Klaucke 17 that is not even in my inventory.
Check out @DubbsMoleman’s Tweet: https://twitter.com/DubbsMoleman/status/1118704686515855360?s=09