This seems to be related to an issue creating backpacks through scripts...

_position = getPosATL player;
_position set [2, (_position select 2) + 0.5];
_b = createVehicle ["B_FieldPack_cbr", _position, [], 0, "CAN_COLLIDE"];

When the pack is created it gets moved away from the player, presumably so it doesn't collide, even though "can_collide" is specified. The pack will then jump between the created position (_position) and the position the engine moved it to, depending on the view angle. If the moved to position is within the camera view it will appear there. If it's not it will appear in the original location. This seems to only be an issue with backpacks. Changing this to create a crate, for example, acts as expected.