First off, re: Horizontal Tag Panel looks very different since v2.87c, thank you Florian for making adjustments to allow deeper Tag Panel customization. Most appreciated!
Completely unrelated, I recently had an album with the same issue described here:
Before I thought to edit the files with foobar or another tool, I was able to resolve this by converting the files to WAV then back to FLAC.
This is not a big deal since I have a work-around, but I am curious as to why Mp3tag cannot completely remove or overwrite the corrupt tag info (no settings for removing FLAC tags specifically).
Please let me know if you'd like me to link you to one of these files.