Why are my dumplings made of evil?
Spoiled buttermilk wouldn't give a metallic aftertaste, but I wouldn't expect old baking mix to do so either. 

I would suspect that your box of bisquick is either contaminated, or you may have gotten a bad box. Sometimes manufacturing processes don't go right, so it might be that your box got far too big a portion of baking powder, or some other component of the mix. In the manufacturing process all the ingredients are supposed to be well mixed but it isn't unheard of for a clod to make it through the process intact. I'd throw the box out.