iOS devices send incosnsistent Track metadata over bluetooth. I've tested an android device with this same A2DP sink and all the correct metadata consistently appeared. On iOS however, the first track streamed with all the correct track meta-data (Artist, Album, etc..) but every subsequent track only comes with "NumberofTracks", "TrackNumber" and "Duration". I've tested both the iPhone 7 and iPhone X and the pattern is the same.
I've combed through the raw data available in dbus and I am pretty certain that the proper track metadata doesn't ever get transmitted (see below).