I am new to this app, but as I have recently moved from managing my library in musicbee, to entirely managing it in Virtual DJ, I wanted a more robust tag editor. While Virtual DJ seems to read (and edit) the tag data on OPUS files, I can't get MP3Tag to do the same. I will add that VLC doesn't seem to be able to read that tag data on these files either, so not sure what is going on.
Thanks for the example file. I can load the file with Mp3tag using the same version and add inspect its properties. It only has ENCODER and LANGUAGE tag fields (visible via the extended tag dialog at Cmd+T) and adding new fields like TITLE works without problems.
What happens when you load the file via drag and drop? Is it listed in Mp3tag's file list? Can you then change its tags?