Capture One is tricky with sidecar files. Have you tried using a manual sync on a single image, to test if C1 can actually process the XMP file? (Image > Load metadata or Image > sync metadata)
If that works, but the automatic sync doesn’t work, it’s likely that the background processes during import didn’t complete. C1 doesn’t always warn about that, it seems. You can then manually select all images you’ve imported and use the same manual sync to force C1 to read any sidecar files.
Hope that helps!