My home has multiple Google Cast receivers (3 x Google Home Mini, 1 x Chromecast Audio, 1 x Chromecast Ultra). All four of the audio devices are in a casting group called 'Home Group'.
For a while, the Google Home Mini in the bedroom was not appearing in the list of devices to select within Spotify. However, I could select 'Home Group' and the audio would still cast to the bedroom that way. Rebooting devices (iPhone, Wireless Access Point, router, receiving audio device) does not resolve the problem. Performing a factory reset of receiving audio device does not resolve the problem either.
I note that iOS apps "TuneIn Radio", "SoundCloud", "Mixcloud" do not exhibit this problem from the same iPhone, whereas the app "Casts" (a podcast app) does experience the exact same problem as Spotify. Perhaps Spotify and "Casts" are using the same component library or SDK for Google Cast which has bugs?
Interestingly, Chome browser on Windows 8.1 laptop can see all Google Cast receiver devices and cast to the missing device via the web player (https://play.spotify.com). Then the missing device reappears within Spotify (iOS) and disappears again when the web browser disconnects/stops casting to the device.
This problem has existed for many months. However, what makes this problem more strange is that recently the Bedroom Google Home Mini has become reliably visible within Spotify (iOS) and instead the Kitchen (Chromecast Audio) device is now the consistently missing device within Spotify (iOS). Again, iOS apps "TuneIn Radio", "SoundCloud", "Mixcloud" do not experience this problem and can cast to all devices, but "Cast" podcasting app has experienced this change too and can now see the Bedroom device but not the Kitchen device.
Finally, a completely separate iPhone7/iOS11.3 on the same Wi-Fi network is also experiencing exactly the same problems described above with Spotify (iOS) even though it is using a separate Spotify subscription.
Hope all this troubleshooting information is useful.