I have been playing round with Mp3Tag 2.48 for a few weeks, and have found something strange with the way it saves tags on Flac files.
If I click on the artist pane to edit it (i.e. main right hand section of screen), and then click outside when finished, Mp3Tag automatically saves the tag, but replaces the "Artist" field with "artist". The same thing happens when clicking in the album pane or the title pane ("Album" becomes "album", "Title" becomes "title" etc.)
If instead I click on the save icon to save the tag of just that Flac file, the fields are renamed to start with a capital letter. (However, clicking on the save icon with multiple files selected does not put the capital letter back again.)
This can be seen by looking at the tag data in a hex editor, or with certain software such as dbPoweramp which actually displays the field names.
A previous poster seems to have discovered this bug, but it was moved to "no bugs":