What I've noticed with issue's like this is that the server thinks your position is somewhere other than where you are currently standing, ive had a number of instances where I'm in a house and drop my weapon to pick something up, and it "dissappears".

When that happens I relog into the server and im spawned into a separate location, whether thats outside the house or 100 steps before i entered the house. Whenever I spawn in the weapon i dropped that "dissappeared is sitting right at my feet whenever i relog like that.

I had a similar issue, two deer frozen in place, I shot both of them, saw their body drop, by the time I reach the bodies they are gone, dont know if its related but two more deer popped up within 50 meters and ran off once i got nearby the location of where the bodies were suppose to be.

take off clothing, don't wear a winter coat in the middle of the day, leave 4 slots in your backpack to store your winter coat and wear a 4 slot tshirt.

same issue occured with me when picking up an MP5 from another player, unable to shoot or aim with the weapon until I logged out and logged back in.

MP5 had all attachments as well as an ACOG scope.

do you use SLI or crossfire? Im only asking because maybe it could be related to that. or graphics settings

I think that this issue is partially lag related, I have ran into this issue several times, I would drop an item and it would "disappear" but really it has dropped 30-50 meters behind me, almost as if the server thinks that my position is not where I'm at at that current moment.

I have not had any issues with items falling through the floor or clipping through walls, however on a number of occasions I've had to backtrack to locate an item that I dropped.