With the release of iTunes 12.5, Apple added support for metadata fields to make it easier to organize classical music. New metadata fields were added to the iTunes library for Work Name, Movement Name and the Movement number.
In m4a files, the new support was in our opinion implemented in the best way posible. The new metadata is stored in the files and iTunes reads and writes the data. Unfortunately, no "file" support was implemented for other audio file types. This means that the metadata associated with the new fields could be retained in an iTunes library but was not read or written from the actual audio files. As the new fields were mostly supported by the iTunes AppleScript interface, the values could be pushed and pulled.
In iTunes 12.5.2, Apple made a fundamental change which will affect the iTunes libraries of many users. The Work metadata is mapped to the Grouping field in supported ID3 based audio files. (ID3 TIT1 frame). This data is read and written. What this means is that any ID3 based supported audio file (mp3, aiff) will have its Grouping data appear in the Work field. Whenever you edit metadata in iTunes, you will lose the previous Grouping data stored in the files.
Aside from the general loss of data, this issue also introduces a definitive difference in the way metadata is meaningful in different audio formats. In m4a files, Grouping and Work are distinct data points. In mp3 and aiff files, there is now only really Work support. Grouping becomes meaningless as it is constantly overwritten.
Some people will note that you can still manually set the Grouping metadata in the iTunes library. That is true. In fact you can still programmatically set the Grouping data. However, you cannot save the data in the audio files.
For those of you who use Grouping there are really only two possible solutions if you are an iTunes user. The first is that Apple fixes the issue. You can always file a bug report at bugreport.apple.com. The second solution is ..... sorry there really is no decent second solution. In order to guarantee that no existing data is lost, every audio file using Grouping would have to have the metadata moved to a different field. All non iTunes, taggers and players which modify metadata would have to be modified to use a different field for Grouping.
The Yate Resources area has a Preserve Grouping download containing two actions which can be used to preserve your Grouping metadata....just in case.