Thumnail/preview problems

I recently began to notice a problem with Media Pro 1.4.1. I typical load a catalog with new images usually in nef or crw format, rate them, and then use Photoshop 5.5 to make color balance and other adjustments to the raw images. After doing these steps I then save the image as dng with the suffix "mod" before the ".dng" file name ending. I then add the modified image to the catalog so I can compare the original with the modified one. What has begun to happen is the preview/thumbnail of the original raw image becomes the same as the "mod" one. If I reopen the original image in Photoshop I get the original image and similarly for the modified one. Rebuilding the thumbnail changes nothing.
If I modify an original jpeg image, taken the same time as the raw one, the thumbnails remain correct.

Any ideas why this strange event is happening? I am running Apple OS 10.8.4 on a late 2010 MacBook Air.

Many thanks, Bill