I had already written that images lose their rating (stars) via the copy and paste function. Now I can concretize it: It is only with Leica DNG files. This does not happen with my old Sony A7 series files (SONY ARW).
Background: If I copy the settings (LUT/Exposure …) from one image and paste them into another image, then suddenly the rating that I did in Lightroom and is visible in Nitro is gone. What could be the problem?
Lightroom writes the star rating directly into the DNG file. Nitro does not write ratings or edits into DNG files (partly due to a concern about the (very low) risk of data corruption). So there are two places for the data to be located.
So, when you edit the file, Nitro makes an XMP file, but (I’m guessing) the rating is not being copied from the DNG to the XMP. I’ll have to look into it further.