Hi Dennis, thank you for your reply.
If I edit (cmd E) an untouched cr2 file without an existing XMP sidecar file via the Contact Sheet of PM6 (Plus) in Photoshop/ACR and crop it there, then ACR creates an XMP file. Back in the Contact Sheet, the crop is not visible. The crop is visible after a restart of PM6.
But if I tag (via shortcut T) an untouched cr2 file first in PM6 (Plus), PM6 creates an XMP file and if I crop this cr2 afterward in ACR, the crop is directly visible in the Contact Sheet of PM6.
It seems, that PM6 doesn't show a crop immediately in the Contact Sheet if a third-party app like Adobe ACR creates the XMP sidecar file of a RAW photo. I never had this issue with PM5.