I've tried this on four seperate phones, using three versions of Android, and it happens on all of them, so it's 100% a Spotify issue.

When using Spotify on Android Auto, occasionally (around 20% of the time) when you plug your phone into the Android Auto head unit, audio playback will not start (or resume from previous playback.) It appears to "play", but no audio can be heard. Audio can be heard in other Android Auto functions, so it's not the phone, Android Auto or the head unit.

In addition to this, if music is playing and the phone is unplugged (as in, when getting out of the car) Spotify continues to play music and must be manually paused. Google Play Music, Deezer or Tidal does not have this issue.