Dunno if there's anything I can do about this, but in the continuing annals of Android Auto being buggy, today I saw it fail to resume playback after I (if I recall correctly) stopped the car, disconnected the phone, reconnected the phone, and restarted the car.
At first, nup didn't even show up in Android Auto after the car restarted (is #53 back?). After I unplugged and replugged the phone, nup's default home screen with the search presets was shown instead of the previously-playing song. I chose one of the presets and Android Auto just sat there with a "Loading your selection..." message (I think that all of the results were new songs that I hadn't synced to the phone). After I went back and chose a different preset, the results actually played.
...
04-12 14:08:52.178 D Player$FilePlayer Finished preparing android.media.MediaPlayer@29d5f19 (5910974510923776.mp3)
04-12 14:08:52.181 D Player$FilePlayer Setting android.media.MediaPlayer@29d5f19 (5910974510923776.mp3) volume to 0.654
04-12 14:10:35.777 D Core Transiently lost audio focus; pausing
04-12 14:10:35.781 D Core Pausing player
04-12 14:10:35.783 D Player Pausing android.media.MediaPlayer@311bc8a (6538090202005504.mp3)
04-12 14:10:35.783 D Core Saving playlist with 100 song(s) (index 8 at 1m 45.053s)
04-12 14:10:36.490 D CoverProvider Got request for 9f038034-8936-409a-bd92-d2fe20565321.jpg from com.android.systemui
04-12 14:10:37.044 D Core Lost audio focus
04-12 14:10:37.130 D Core$_playerImpl Media session request to stop
04-12 14:10:37.132 D Core Aborting playback of 6538090202005504.mp3
04-12 14:10:37.133 D Player$FilePlayer Closing android.media.MediaPlayer@311bc8a (6538090202005504.mp3)
04-12 14:10:37.874 D CoverProvider Got request for 9f038034-8936-409a-bd92-d2fe20565321.jpg from com.android.systemui
04-12 14:14:23.390 D MediaHelper Request for root item
04-12 14:15:04.874 D MediaHelper Request for root item
04-12 14:15:06.113 D MediaHelper Request for children of "root"
04-12 14:15:06.115 D MediaHelper Request for item "root"
04-12 14:15:06.178 D MediaHelper Request for children of "presets"
...
04-12 14:15:10.809 D MediaHelper Request for children of "albums"
04-12 14:15:10.819 D MediaHelper Request for item "albums"
04-12 14:15:22.381 D Core Request to resolve item pr|4 ("null") for set from com.google.android.projection.gearhead
04-12 14:15:22.383 D Core Saving empty playlist
04-12 14:15:22.384 D Player$FilePlayer Setting android.media.MediaPlayer@cbbe6cd (6547573489795072.mp3) volume to 0.343
04-12 14:15:22.399 D Downloader$download GET /query?shuffle=1&minRating=4&maxPlays=2&onlyIds=1
...
04-12 14:15:22.402 D Player$FilePlayer Setting android.media.MediaPlayer@29d5f19 (5910974510923776.mp3) volume to 0.423
04-12 14:15:23.471 D Downloader$download 200 (OK) for https://.../query?shuffle=1&minRating=4&maxPlays=2&onlyIds=1
04-12 14:15:23.485 D SearchKt Server returned 18 song(s)
04-12 14:15:23.488 D SongDatabase$query Running "SELECT s.SongId, Artist, Title, Album, AlbumArtist, AlbumId, DiscSubtitle, Filename,
CoverFilename, Length, Track, Disc, Date, TrackGain, AlbumGain, PeakAmp, Rating, Tags
FROM Songs s
WHERE s.SongId IN (6199351284072448,4696501919940608,4802591101812736,6273773630128128,6230734903705600,5176543770836992,5916238674919424,6242483182764032,5135825668931584,4839796230848512,5149761227194368,6312702811045888,5174622779604992,6247171114401792,4828203199758336,6316096414023680,6322281502474240,5094262599319552)
ORDER BY RANDOM()
LIMIT 250" with args []
04-12 14:15:23.511 D Core$_playerImpl Media session request to set 0 item(s)
04-12 14:15:23.515 D Core$_playerImpl Media session request to prepare
04-12 14:15:23.522 D Core$_playerImpl Clearing stopped state
04-12 14:15:23.529 D Core$_playerImpl Media session request to play
04-12 14:15:23.537 D Core Requested audio focus; got 1
04-12 14:15:23.537 D Core Unpausing player
04-12 14:15:48.040 D MediaHelper Request for children of "albums"
04-12 14:15:48.048 D MediaHelper Request for item "albums"
04-12 14:16:17.845 D Core Request to resolve item pr|1 ("null") for set from com.google.android.projection.gearhead
...
...