Plays on car stereo then reverts to phone

[AndrewAndroid]

Plan Premium Country Austria

Moto G7 Plus XT1965-3

Android 10 QPWS30.61.21.11 security patch level 1 June 2020

Spotify 8.5.66.1002

Connected to a brand new out of box Kenwood DMX7017DABS Mechless Android Auto compatible double din head unit via brand new out of box Anker Powerline+ USB C Cable to USB 3.0 A, Nylon Charging Cable 0.9m

When I first start off on my commute in the morning the phone connects normally and Android Auto starts up with the Spotify interface at the bottom and I can start to play media normally. If I stop and start the car all other apps continue to operate normally (eg Maps, Google Podcasts, Google Play Music, hands free phone) but Spotify will only play music out of the phone speakers and will not play over the car stereo again until the car has been turned off for about an hour or so. Although the music is playing over the phone speakers the Bluetooth symbol indicates that the phone is connected and that it recognises the audio device for audio output (and can clearly operate in that mode because the other audio apps are successfully outputting over Bluetooth. Disconnecting and reconnecting does not change the situation. Reboot, uninstall, reinstall carried out already.