I think your intuition on the seals is a good one
The only other explanation is that some of your previous oil changes were a little too long and left deposits in areas you can't see. The higher detergents in the HM oil have gotten to them faster.
I was surprised how black M1 0w40 was after 6k oci / 30k total after being dealer serviced with presumably the same oil at 7k, then 17k, then 24k. The current fill at 6k is nowhere near as black.