I've found that if I add tracks to "My Library" using Google Play Music All Access, and play them, I get a play count added to the tracks the same as I would on any other track. However, if I now delete the tracks from my library (but still have access to them as it's all access), then I lose the data associated with the tracks - play counts and ratings. This is especially annoying as to force tracks to download to my device I need to add them to my library - it's automatic.
Am I misunderstanding the behaviour of this feature? The situation seems to be that if you ever add a track to your "My Library" (including unknowingly if you elect to download the track) then remove it, you lose the history associated with it.