That’s certainly odd. The images are stored in the Finder and you add a keyword in Nitro but it’s not saved?
Is the XMP data you put in this post from the Nitro XMP sidecar file or is it from digikam or something else?
Please try the following:
1. Open Nitro
2. Select a file that doesn’t have an XMP sidecar
3. Rate and add a keyword
4. Go to the next image to make sure it’s written out
5. Open the XMP sidecar in TextEdit or something – is the keyword present?