We're not sure why this is happening. Its possible that collecting some files from you will allow us to investigate further:
The catalog database itself could be useful, the location of which you can find in the Manage Catalogs window
The catalog state file might also be useful, that would he located here:
C:\Users\<UserName>\AppData\Local\Camera Bits, Inc\Photo Mechanic\catalog\state\cat_state.sq3"
Quit Photo Mechanic and then zip up these files to send to us. There will likely be a "proxies" folder in the catalog location, you don't need to send us this. Send them to support@camerabits.com
Alternatively, when there's a unique problem like this one, sometimes doing a screen share helps reveal things that I didn't otherwise think to ask about, so we can go that route if you'd like (I can get the requested files at that time as well if we do that).
Best regards,