[kk2002s]

I have had magnetic drain plug last 3 changes. I have always seen a thin film of black slug, no specs as OP's show
I tried to see if anything from the plug would stick to a magnet but that doesn't work.
What I did find by accident the last time was I rubbed the film off on to tissue paper and when I put a magnet to the film stain, it was attracted to the magnet. I think there was a tread about this before but I could find it with search.

I haven't cut open my filter yet as Expansion tank replace took priority

I hope you find your sump clean. I have done that a couple times and that does give me some piece of mind to find it and the filter debris free