I tried to pick up an item (long range scope pristine) and place it in my backpack.

I removed the backpack in order to pick it back up again, opening to clear my inventory issue. It would not remove... then about 10 sec later it removed and vanished.

I took off backpack in the first place: as sometimes this lets me add things to the backpack or clothes if they can not be placed in the open slot on the inventory item (eg shirt/pants/backpack)