Background: I decided to clean my library by removing all ID3v1 and ID3V2.3 tags then updating all to ID3v2.4. This initially seemed to have worked without any major problems but after I ran 'Analyze Audio' then 'Show Files with Missing Tags' I noticed a recurring problem with several MP3 files.
Example - one track in a Nick Cave album did not have the correct tags listed, i.e. Album, Artist, Genre were blank. So I updated it with the missing tags. After adding the missing tags, Analyze Audio showed that the album needed analyzing again. After repeating the analysis the same track was missing the tags that I just added.
I tried different methods of adding the missing tags but each time I did, Analyze Audio told me the album needed analyzing again which then wiped the tags again.
I eventually solved the problem by switching to ID3v2.3 tags.