Hi. I wanted to add the composer tag to a number of albums encoded with flac. This I did with MP3TAG. Checking things over, I noticed that some files after adding the composer field were empty. I added the composer again. I re-checked, again empty. I used Finder's "Get Info", and the composer was present in the Composer tag, at times duplicated. Yet, on MP3TAG the field continued to be empty. I've attached an image of the same. I suppose that I could forego adding the composer, but it's a useful indicator for me.
It looks like the composer field starts with empty text and possible linebreaks (indicated by the dots ...). Can you check the contents of the COMPOSER fields via the extended tag dialog at ⌘T and see if the data is correct?