well the thing goes just like KillzoneKid described. When player drop its backpack, so that backpack will be placed into the ground via groundWeaponHolder(Entity) which means that engine will lost an information about dropped backpack as a standalone world entity and whole simulation and render behavior will be assigned to a created weaponHolder. So if you think you see a backpack in front of you - you see nothing but the one of dummy proxies(from weaponHolder.p3d) replaced by a backpack shape model.
You will be still able to add/remove items into backpack through assigned variable, but you definitely won't be able to work with them as a standalone entity(moving, deleting, etc..)
this behavior has been implemented as a part of further design document(section: interaction with a content placed on a ground and/or crate). In fact, before "last update(your word code34)" the behavior went against design document(backpack was placed on a ground directly as an entity) so I have just fixed the bug :-)
code34: JFI - I really don't like the way you're speaking to another community members. Let me be quite personal.. well I'm not so well informed about community peoples but from my point of view: peoples like KillzoneKid, Samatra, etc. have already done many nice things, workarounds and bug reports so I'm hardly sure that KillzoneKid is not unskilled noob or troll. For the other hand, the only thing I know about you is you are not so polite to others ;-) Just my two cents
but gets back to a real problems: this is not a bug, but the design decision. Every engines have theirs limitations and this is one of them
Maybe I can add some kind of scripting command like [GetEntityOwner()] which returns owner weaponHolder so you will not have to use any workarounds