This is pretty strange. I already tried restarting Rhino, and reloading the file, but…
It appears that Cycles doesn’t quite know which resolution to render at. This cloth texture was as sharp as a tack on my laptop today, and on this desktop computer just last night. Yes, this was a 4k screenshot.
Bonus strangeness: the resolution breathes wavering in sharpness : O
"Tis the work of the devil! LOL!