Here's one that's been driving me crazy: The other day, I noticed songs on my phone's "Thumbs Up" playlist, songs that were both bought from Google Play or from elsewhere, were greyed out on my phone.
This seems to be a problem with Google auto-matching songs to their own songs offered on the Play Store. The songs that are greyed out and were uploaded have an option to "Fix Incorrect Match", but songs that were purchased from Google are greyed out anyway. Songs that can't be matched are left alone. This is also only limited to the "Thumbs-Up" playlist.
(Click the image for full-size)
This issue does not persist on Windows and does persist on Android regardless of internet connectivity or playlist downloading.