Files falsely showing "no images in folder" on SSD

I am sure this has been pointed out before. Sometimes strange occurrences that pass an integrity test when the catalog is backed up, require the verify and the repair process to correct problems. The Integrity test apparently just verifies that the catalog can be opened.

After installing version 16.1, half of one folder's* subfolders displayed the message, "No Images in Collection" instead of displaying the thumbnails of files in those subfolders. Clicking on the corresponding albums in the library showed the corresponding thumbnails and they could be located in Finder. Moving some new files from a  catalog album to an affected subfolder resulted in them being moved to that subfolder on the hard drive, but the subfolder count was not updated. No other folders were affected. 

The problem seems unrelated to version 16.1 and is related to a catalog database. The same issue was present in the backup copy I made before upgrading the previews. When the database is backed up, the Test Integrity process is not comprehensive. When I ran the Verify Catalog it failed a full verification check, showing: "Checking for catalog folder collections without a project FAILED! Database content error."

I then ran Repair, which fixed the problem. What caused the problem is a mystery. 

It is probably worth running Verify Catalog from time to time.

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* I am using the Capture One nomenclature where Referenced files are in "folders" are on a hard drive. They are displayed in the "folders" section below the Library section in a catalog.