This bug applies not only to mystic shot, but to any item that procs based on the "central" pellet of an attack. Headshot booster, Headhunter, and Restorative Shot are all affected as well. It seems like because calico's shotgun spread has no pellet in the center, the central pellet was set to be the one at the very bottom. It's unclear if this is intended, but it certainly is unintuitive and makes especially the headshot items very difficult to use.