Sorry my bad, not really a RB bug.
It's caused because the comment is not updated in iTunes/Music system. So when i edit the comment in RB, next time RB loads it fetches the data from iTunes (where the ID3 is not updated yet in the framework) so the comment turns blank again in RB.
The only way to keep comments in sync and not disappearing are:
- Either edit the comment in iTunes/Music and not RB.. then RB fetches the comment on the next load
- Or edit the comment in RB but then you need to click on the track in iTunes/Music before RB loads next time. By clicking it the ID3 is updated in iTunes's database/framework.
Of course it'd be cool this would be less tricky, but not sure if Apple's framework allows writing to iTunes/Music database... So when you change something in RB it would automatically update iTunes/Music.