I have been a Spotify Premium user for a long time now, and have been using Chromecast Audio's to play music through my house for over a year. In the last month, the ability for Spotify to find the Chromecast Audios has become very unreliable. Specifically, if unless I reboot my phone, Spotify does not find the Chromecast devices. And does not seem to update the list of Chromecast devices if I power off one.
I have uninstalled/reinstalled Spotify, verified the issue on another device, power cycled all of my chromecast audios, and rebooted my wifi router. None of these things seem to help. The Google Home app sees the Chromecast devices and changes to them without a problem.
I have also tried killing the spotify app, turning on and off the wireless, etc. My phone upgraded to Nougat today, and my biggest hope was that that would resolve this issue, but no dice.
Spotify is becoming very frustrating. And that is not even mentioning that you still can't configure Last.fm scrobbling for Chromecast, which has been a long term request from me and others.