I am experiencing this issue with PM6 Build 3889 (Windows 10) also.
After sending an image to photoshop to edit, and then once im finished and the image is saved out in the same folder as either jpeg or tiff. Then upon returning to Photo Mechanic it does not pick it up and display the edited image.
Manually refreshing the folder, closing and opening it, or even restarting PM, yields various results, sometimes within moments they are detected, other times a significant time and as I write this the current lot of images I'm working on are just outright refusing to show. and yes 'Rescan on resume' is ticked.