Can someone please help me out with this?

During infinity war Black Panther issues Bucky with a new arm ‘where’s the fight’ ‘on its way’.

Obviously the arm is not a part of his body, merely an attachment. But during the snap, Bucky along with his prosthetic arm dissolves and his gun drops to the floor....... Surely like the gun, his arm should’ve stayed ‘undusted’ as it was not a part of his body!

Anyone know if this is an error or is there a reason for it?