In apple music add an album to your library. If you add an album that release date doesn't match actual CD release you will notice the following behaviour (as example take first Dead Can Dance album). It was released in 1984. If you add this album to your library and look at your library right away it will show 1984. however when you download the songs locally the year will be overridden and change to 2008 (That is particular CD release date). It happens for big number of albums.