Synchronise "Remove missing photos from catalog" showing files which are not missing!

I've been using Lightroom for many years.

I'm on Windows 10.0.26100.4770 and LR Classic 14.4 build 202506051112-5918896a with Camera Raw 17.4.

I often import images using Photomechanic to my PC and then use "Synchronize Folder" to import them to my catalog (one big catalog for all my images).

Recently I've noticed that the "Remove missing photos from catalog" option shows some are missing. When I hit "Show Missing Photos" a moment ago I see nine images apparently "missing" from the catalog.

However if I right click those images and tell it to "Show in Explorer" the image is brough up just fine and I can view it in Explorer. Indeed I can view it with Photo Viewer in Explorer or open it in Photoshop. No mention of metadata issues, no "warning exclamation marks" on the images in view.

It is as if the images have been flagged as missing even though they are not.

As an experiment I removed one of them from the catalog ("remove photo") and then did a synchronise folder again. It told me there were 8 missing photos. It found that one image. It imported it successfully. Then I went back to synchronise again and looked and there were 9 missing photos again. And of those 9 missing photos - yup, that one was back in the list that were missing!

I selected all files that were "not found" and did a Metadata>Save To Files. Then I tried a sync again - still 9 apparently missing files. All of which I can find and view.

Any bright ideas?