Starting today, Discovery+ ads have left a dark area at the top of 4K Smart TV on all Xfinity channels. TV checks out OK and dark area does not appear when using TV Plus, Netflix, etc. So, it does not appear to be a TV issue, but is a Xfinity issue related to these new ads. No problem exists, however, on non-smart TV's.
Not really an acceptable answer, but, miraculously, or by coincidence, the "dark" area disappeared about a day after my inquiry. Maybe someone viewed my inquiry and resolved the problem, be it xFinity or Discovery, and it has not been a problem since.