Odd Skjaeveland, thank you for your reply!. Using this information I was able to get it to work.
It hadn't occurred to me (obvious now) that changes in CaptureOne (C1) are recorded in its own files and not automatically recorded in the side car files. You are correct, clicking "Sync Metadata" in CaptureOne works both both ways: Both getting C1 to read changes made in PM6 from the side car file and getting C1 to write to the side car file so PM6 can show the change.
I'm not sure what "Load Metadata" is good for over "Sync Metadata".
It still does not work for .dng files, but I think this may be a limitation of the .dng files?
Ric