When inquiring on a mp3's detail properties, there is text in Mood and Group Description fields. They don't show up in current Mp3Tag v2.90a. I can't blank them in mp3tag but can blank them manually in detail properties. I'm using Windows 10 64bit.
Try adding something in these fields via properties and see if they show up in Mp3Tag. I have Grouping and Mood fields displaying in Mp3Tag.