Material tags don't seem to find their host correctly on first placement and usually require a leader or a slight shift of the tag to populate. Second material tags tend to fall off their host with the smallest change unlike other tags, which is why I generally avoid them, but our firm does use them for glazing types for example. And while I understand why there is no tag all not tagged option as that could be very messy in most views, perhaps limiting to selection first for tag all not tagged would make repetitive tagging somewhat easier.