Both chromecast and Spotify app is updated several times for iPhone since this issue, but still no fix!:(
This started happening sometime this month (I don't use Spotify on Chromecast that often): used to have lyrics / behind Genius content, now just the logo in the black background :-(
Maybe another Chromecast update broke it? Tried rebooting to see if there was any pending update to fix, no dice.
Ok, apparently I'm not the only one in Aug 2019, and Spotify Google is aware:
I'm having the same issue - started this week: