Nothing is wrong, it's no bug, those stairs are always been like that. The one with the driver are a separate model, and if they both fit the airplane, they all have a chance to appear so, it's entirely normal you sometimes seem the ones with the driver, and sometimes you see this model, which has been there since GSX was released.
They are going to be replaced in the next update (only in P3D4.4) with a better looking model and, it might be possible (if we have time, if not, it will be in the next update), we might add a human animation, because I believe that particular model is small enough to be operated by a human pulling it.