First image below. This is a crop of a severely underexposed image further pushed to its limits by gradient adjustments and I forget what else. Profile is Adobe Neutral. Calibration is Version 4 (Current). Magnification: "Fit" (Lightroom 7.4, Camera Raw 10.4, Color Space: ProPhoto RGB 16 bits).
The issue is the red blotching.
Next is the image at "Full": Some blotching gone.
Finally, at 1:1 everything seems copasetic:
A side note: When this image is opened in Photoshop as a Smart Object, the same bad blotch is seen when the S.O is opened in ACR, but it is not visible otherwise; in other words, Photoshop renders the smaller image nicely but exhibits similar display characteristics inside S.O. ACR.
For laughs, here is the image before two gradient masks w/luminosity masking which applied low noise, low clarity, and low sharpening:
It looks 10x uglier blown up.
I'll also note this image was taken on a Fuxi X-E1 several years ago. I'm hoping this isn't some weird fallout from X-Trans sensor rendering.
One other aside: as Lightroom is flickering to render the final image, the blotches appear momentarily. It is as if the blotches are an early stage of rendering, but Lightroom doesn't go to the "next level" correctly.
I realize I might have provoked the Lightroom beast by tortuous adjustments, and this is the only time I've seen an anomaly like this, so it's not a major hindrance - but I thought I'd document it, and fly it by the ninjas here. Is it a bug, or have I lost sight of a fix somewhere?