I should also mention that maple syrup can indeed ferment, especially when not refrigerated, as I once learned to my chagrin. So you may have actually been tasting alcohol after all. One particularly nasty aspect of this is that there’s a buildup of pressure inside the bottle as a result of the gas produced, and the syrup can come fizzing out even if it’s tightly sealed (I was probably lucky the damned thing didn’t explode). I was about to throw the contents out, but decided to heat the syrup and see if the alcohol would evaporate. It did, and the syrup tasted fine again.
Even worse is that mold can grow on top of the syrup once the bottle is opened, so keep it tightly sealed and in the coldest part of your fridge. If it gets moldy, the only thing you can do is throw it out. Considering the high cost of real maple syrup, that’s something you don’t want to do.
BTW, I would assume that the metallic taste in Wisconsin was due to the minerals present in the soil.