I'm not able to reproduce that issue on my end. What version and build of Photo Mechanic Plus are you on and what operating system?
If you choose "Update Folder location" does that work? (you should see a window that lets you pick a location for the Folder, you can just cancel out of it once you see that).
If you want a quick workaround, you could always just click on the folder itself, and then when it opens a Contact Sheet, select all the items and then right-click on any one one of them and choose "Remove Items from Catalog", an then click 'Remove' on the proceeding dialog. Make sure you only have the "Add/Modify" box checked only for the catalog that you'd like to remove the items from.