Hi @GEdit, I believe what you're seeing is not a bug but a side effect of how the XMP metadata system works.
The "Rating" field is not part of the Premiere Pro Project Metadata schema. You're correct that if it were, this would be a bug. That metadata is stored only in the project file.
However the Rating field is part of the "Basic" schema, which uses XMP to store the metadata with the media file, not the project file. For some media formats, that means a .xmp sidecare file will be written.
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