I've made two darker beers recently, a English Mild and a Dry Stout that have both had a smokey off flavor. The only common grain used is Marris Otter as the base grain. Other than that I used roasted barley in the stout and pale chocolate in the mild. I used Wlp002 for the mild and wlp007 for the dry stout.
I've been adding about 2g CaCl2 to mash and boil for total of 4g. Could the minerals react with the roasted malts to cause this?
Could it be yeast phenolics?