I guess MC isn't finding the image, given that the [Image] tag is empty.
Is there anything under the Tag Dump to give a hint? The image I have attached shows the Tag Dump area of one of my FLAC files that has an image inside the file. MC recognises there is one picture in the file. Do your files show that information?
Perhaps the image file is in another tag, or the tag it is in isn't the usual one. Does Media Info give any hints?
From memory there were some changes to allow or recognise more than one image in a file. Maybe MC sees the image, but none of them are set to the default image, or something like that. You might find details in the Release Notes in the Wiki, which might help identify in which version it stopped working for you, and hence when the problem was introduced.
I think Foobar recognises and can display all images in a file, which may be why it is able to show the Cover Art. MC only shows one image stored inside the file.
Do any other tools see the image? Even Windows Explorer should show the image.