I have some music files (.mp3 and .flac) which have unmodifiable tags: After I change the tags, they revert to their original values automatically. It's not related to access permissions (using Linux) as in a same directory, with the same permissions, some files have modifiable tags and other not. Is there such a thing as "protected" or "unmodifiable" tags? Are there solutions to force modification ? Thanks.
AFAIK, there's no such thing as an 'unmodifiable tag' (unless the file is set to 'read only'), however, this situation sometimes arises when a tag is corrupted and/or written in a slightly non-standard manner that another application doesn't recognize.
In general when this happens, you can try using another tagging app, or more universally, use an application to remove the tag entirely, and then recreate it.
e.g. in MediaMonkey, Tools > Advanced Tag Management > Clean ID3 tags