Okay, did it again.
After last night's shave I did my normal 'dry the edge' strop routine.
Checked with the loupe, and there was the cheese again.
I wiped it clean with a dry tissue and did the clean-up stropping. It stayed clean.
The very edge appeared to be clean with the crud farther up the bevel. So, the strop is working, but only at the very edge.
I have to be more diligent with the initial toweling when drying the blade to be sure the bevel is well wiped.
This whole thing caught me by surprise.