Spotify has an ongoing casting issue which isn't present with it's direct competitors. I am able to replicate the issue across multiple devices / accounts / networks and see consistent behaviour which I believe attributes to a bug.
This is on a OnePlus 3 Android device (not tested iOS). Version 7.1.1, latest version of Spotify. Also tested a Samsung Galaxy Tab A (2016) and a Samsung Galaxy S7. All are up to date with their latest OS and Spotify updates. The app has been removed, multiple accounts tested, and it can also be replicated on another network.
To explain the issue / scenario:
I want to cast to my Google Home downstairs, so I click on 'devices available', the list comes up. I select 'living room'. It connects and successfully casts.
The problem starts when you wish to change the casting device.
If you bring up the list of devices again, and select another, for example 'kitchen', Spotify will just sit there connecting forever and not do anything. If you click 'kitchen' a second time, sometimes it will then play on 'kitchen', however it does not close off / stop the previous cast to 'living room'. This then causes all sorts of issues because you have multiple casts open from the same device, and things get messy. Spotify gets confused and the whole thing fails. If you try to connect to a third device, the same thing happens, all **bleep** breaks loose and often you lose the list entirely until you reboot the app.
The ONLY reliable workaround I have found is, if I want to change the casting device and have it actually work properly (say, change from living room to kitchen), I have to pull down the notification drawer during casting, pull down the built in Android casting notification so it displays all the controls, and click the "stop" button to stop the cast. Then, in Spotify, I can select another device and it will connect first time. It doesn't get confused because I am manually making sure there is only one cast active from my device.
Google Play Music does it a bit differently, and it works. Once you are casting to a device or group, if you want to switch casting device mid song, you click the cast icon in the app, it brings up the currently selected device with a casting volume slider, but only one option is available - "stop casting". It doesn't display the device list like Spotify does during casting and allow you to select others. Only once you've clicked "stop casting", you can then cast to another device or group by clicking the casting icon again. So it's a bit more simple, but it works reliably. It essentially makes sure you only have one cast active on the device which requested it originally.
Spotify need to fix the app so that when you are selecting different casting devices during playback from your device, it ceases casting on the previous one. Either that, or do what Google Play Music does and force the user to stop casting before allowing them to select another device.
If Spotify can somehow make it so it "automatically" stops casting from the previous device when another casting device is selected, that would be the best of both worlds and even better than how Google Play Music does it, because it would remove a step and be very slick. But as it is, the functionality is essentially broken and needs fixing!
Currently you have to rely on built in Android controls (if you have them available), and remember to perform this workaround every time you want to switch devices in order to have Spotify NOT break things. Having subscribed to Spotify for a year and then discovering quickly that this key functionality is broken, one of my main uses for it is badly hampered. My girlfriend has already lost confidence with the app and hardly ever uses the casting now, because of it's issues and the confusion it brings.
I can replicate this across three Chromecast Audio's, three Google Home's, and all the groups I have created.
Without meaning to sound abrupt, please do not suggest solutions such as reinstalling the app, trying another account, trying another network, clearing out my Android device, reinstalling, rebooting, resetting my casting devices, etc, etc, etc. This and many other things have already been tried and the issue persists across a multitude of different devices.
Perhaps have a look at how Google Play Music / Deezer deals with casting?
Please see this forum post:
https://community.spotify.com/t5/forums/v3_1/forumtopicpage/board-id/spotifyandroid/highlight/false/...
This was originally raised by someone else, but I have posted some of the information above within the thread prior to raising this issue, to try and get some attention.
If you need any information, videos, etc which demonstrate the issue, as long as Spotify takes the bug seriously and adds it to an official tracker or is willing to report it to the devs, I will take the time and provide anything you need which helps resolve the issue!
Casting devices such as Google Home's, Chromecasts, etc are becoming more popular, and with Google opening up to third parties to create their own speakers with Chromecast / Assistant built in, this issue will only become more prevalent with time.
Thanks