This is not a bug. Carcasses have values in the game. Dealing with them ingame eg:skinning or butchering a carcass. allows the game to account for weight loss, skin loss etc. Trying to deal with them "outside" the game can be problematic in various ways. What you're seeing is "as long as you hold the object" the weight is tracked by your character (essentially). Once you drop the item on the floor it reverts to "It's" defined weight.