Seems like the authorization Google Maps gets to control/play music from Spotify is lost after X hours on iOS.
I use both Google Maps and Waze and both experience the same problem. In the morning commute, neither app is able to start playing music. For example, in Google Maps if I click Continue playing button, it switches to the Spotify app to get an authorzation, goes back to Google Maps, but button still says "Continue playing" and music doesn't play. If I force close the Spotify app(swipe up to middle of screen, find Spotify, then swipe up in the app preview), then go to Google Maps and click Continue Playing, then it starts working. It works for the whole drive (around 1 hour). At about 4PM about 8 hours later, when I open Google Maps or Waze, and click to continue playing music, it doesn't work. It switches to Spotify, says "authorized" or something like that, jumps back to the Google Maps app, but it doesn't play the music. Button in Google Maps still shows "Continue playing". As mentioned before, I have to force close Spotify app, then click on Continue playing again, and then it works.