for me it appears to be a random occurrence. When it happens it'll look like the item is still on the ground but I know it's in my hands. because i can't put anything else in my hands, swap or drop anything. I have to log out entirely. even when i exit the server my character standing in the lobby doesn't show that i'm holding any items in my hand. but if i log out the game, back to the steam page then start the game up, i can finally see the item in my hands.

Same issue with me. but it only happens once in awhile though when i'm trying to melee a zombie.