Keeping XMP Files Updated for Ratings & Color Tags

I need to support the following workflow:

• Shoot tethered C1 Sessions
• Review images with subject and rate images using Stars and Color Tags
• Output lossey compressed DNGs with Star and Color values embedded as IPTC tags

Given that C1 can't output lossey compressed DNGs, I need to use the Adobe DNG converter. It will pickup the star/color data from XMP files, so I need those RELIABLY kept up to date when changing ratings/color tags in C1.

When I set C1 to 'Full Sync' for the auto sidecar files, I am having lots of issues with values changing. If I select an image and change the star rating from 3 to 2, select a different image, and then select the original image the rating value will change without me doing anything.

I also tried leaving the sync to "None" and doing Image - Sync Metadata from the menu, but I am now seeing issues where I lose all star/color rating values when doing that. 

Is there a reliable way to have the XMP files stay updated without risking losing values in C1? I'm trying to avoid doing something like Export Originals and running the DNG converter against the output folder as that is just an additional step for something that SEEMS like it should work (XMPs staying updated).

Thanks!

jon