When I save the tracks for a release, none of the track specific performer tracks are being saved.
In the screen shot below, I have already tagged the release with Picard 2.8.5 and save the files. When I open the tracks again, the track specific performers are missing - see below
This looks correct to me. Both the original and new value columns show the same data. “Original” is the data as loaded from the files, so this indicates the data was saved properly. If the data would not have been saved the original values would be empty.