the problem is only when you change rating in Fast Raw Viewer? only the file changed will have the problem?
Bugs which have been reproduced by a second user or the developer, or features to do
i can reproduce, i start XnView MP & FastRawViewer on same folder, I change a rating with FastRawViewer, it doesn't touch the file but .xmp, and XnView detect only changes on .jpg not .xmp, so rating is not updated. As workaround, you need to use 'Update catalog from files'