Most (other) applications do not support XMP sidecars for TIF/TIFF files.That's why XMP support was not implemented in FastRawViewer versions prior to 1.7.0
In FastRawViewer 1.7.x you may enable XMP support for TIFF (also for HEIC and for PNG) files in Preferences - XMP (and, again, ratings/labels assigned may not be visible in other apps).
Most (other) applications do
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Alex Tutubalin/FastRawViewer team