This might not be a bug - but sure is odd.
Changes to tags in Ogg Opus makes the "Tool" field (shown in for example foobar2000) blank. Before edits to the file in mp3tag v2.58, this field shows
"Tool: libopus 1.1"
and after edits it shows
Checking file with MediaInfo before edits in mp3tag v2.58 shows
"Writing library : libopus 1.1"
and after edits this line is gone.
Is it mp3tag that removes this information or what is going on?
I use it to keep track.