Until this gets fixed, you can bypass this bug by changing the color of the torso, left leg, or right leg.
The change can be extremely minor, such as a slight hex code alteration in one of the legs (EX: The same effect as shown on the right side of the image will occur if one leg is #ffffff, and the other if #fffffe. Visually, there is no difference, however, 2D clothing will appear when trying on an item.)