Hi Rien,
I tested your workflow on my W10 PCs and so far the Preview window's auto advance when adding a color label, rating or tag is working correctly. When you add a label in the Preview window and Photo Mechanic does not auto advance to the next image, does the label get applied to the matching thumbnail on the contact sheet? If Photo Mechanic is unable to write the label, then that could be why it is not auto advancing in the Preview window. What type of files are you working with and are they on an internal or external drive?
Thanks,
--Bob