xmp:Label="blue"
Are you sure that your label names in Photo Mechanic and Lightroom are *exactly* the same, including the case?
These labels are case-sensitive. When I change a label of "Blue" written in Photo Mechanic to "blue", and then ask Lightroom to read the metadata from the disk, the color label box in Lightroom turns white, indicating that it doesn't recognize the color label.
I have no idea how this could be working in one direction and not the other, or why it could take hours. I'd have to guess that the issues lies with the Lightroom catalog and you should create a new one as a test to see if the same issue occurs, but that is about as much advice as I can give for that.