The pros have known this for a long time. When using spices, rubs and other ingredients they have a tendency to interact with various types of metal bowls and containers. During this interaction, the rubs and sauces will pick up a slight metal flavor if use with reactive material such as aluminum, copper or other common type of mixing bowl.
You should try to use glass or plastic to both mix and store your seasonings so that they will not pick up any undesirable flavors. If you look on the seasonings situated on the grocery store shelves they're all packaged in either glass or plastic. Likewise when you store your seasonings for reuse on another day always use non-reactive glass or plastic.