No metadata in flac files after cover has been added

[ohrenkino]

Windows software has difficulties with pictures in flac files that are larger than 4MB (well below the standard limit)

see e.g. here:

Metadata only show up in Mp3Tag

One of the possible causes for Windows File Explorer is "too big cover art". Where "too big" is something about 4 MB. Could you try to reduce the cover size to something about 2MB and then try if the windows file explorer show metadata from such a flac with reduced cover art file?