I imported the file you provided, then did "Cover Art > Get From Internet...". An image was found (actually two, I picked the first). After saving tags, the file contained invalid image data. The MIME type is saved as "???AlbumArt" which is not recognizable. The actual image data do not seem to be of any common types either (MC defaults it to jpg, and no image viewer can open it).
Unfortunately I did not save a copy of your original file before trying it. Now I can not reproduce it and thus can not figure out why an invalid/corrupt image went into the file. Any subsequent attempt at doing "Cover Art > Get From Internet..." on the "tampered" file will actually produce a valid embedded image in the file.
Can you upload another sample file for me? Or else you can just try setting cover art on those files again, using the latest MC version, and see if the problem goes away.