I think the main reason could be low humidity , and because its winter
Try keeping your slings in a controlled climate , I've kept all my slings in a bigger enclosure with a heat pad and a huge water container , this keeps the temps and humidity up , even if your Ts container is dry the humidity will still be fine

When your T molts again it will regenerate the lost leg ,but it will be smaller compared to the other legs , by the 2nd molt its leg will be the same size as the other legs ..