Hi,
In your image, we can see a sidecar file which would suggest the file has been (or once was) imported in MC.
Last time I had this kind of issue, the file was actually imported, but just not tagged as I would expect and did not show up in any of my main views (it was filtered out based on views settings).
What you could try is adding a pane view that lets you sort by file location, but does not use any other filter to select file. Then you should be able to see whether there are actual files in MC from the incriminated folder and eventually correct the tags to have it show up in your other views.
Not sure this is the issue, but it might be worth giving it a try.