Hi David,
I am not able to recreate the rating problem you are describing on my Mojave Macs. I think resetting Photo Mechanic's preferences could fix this problem. With Photo Mechanic running, go to the Photo Mechanic menu and then click the Preferences option. Now click the Export button at the bottom left of any page. A list of selected items will open next. Click the Export button at the bottom to save the PM_Export.PMX file to your Desktop or Documents folder. Now quit Photo Mechanic.
With the Finder running, open the Library folder from the Go menu or use the Command-Shift-L shortcut. Next open the Preferences folder and locate the com.camerabits.PhotoMechanic folder. Renaming this folder will cause Photo Mechanic to create a new one the next time it starts. Just append the date to it. Eg, com.camerabits.PhotoMechanic-4-2019. Close the Finder window.
Start Photo Mechanic and try the same the same rating, cropping and then re-rating workflow. If everything is working correctly, then go back to the Preferences to Import your old settings. Click the Import button left at the bottom of any page. Locate the PM_Export.PMX file you saved on the Desktop and then double click the icon. The same list of selected items should open next. Deselect the Preferences:Caching entry towards the top of the list by Command-clicking its entry. Click the import button at the bottom to continue. A message dialog will open next asking if you want to merge the file's settings with the current settings. Click the Yes button to complete the Import. Photo Mechanic will need to restart. Test your same workflow to ensure the problem did not get restored by Importing the preference settings.
--Bob