Tried posting this here:
Thread was almost immediately closed rather than considered, with directions to post it in "Other Issues" which nobody is allowed to post in - or at least I'm not able to.
Please consider reading this thread and notifying spotify engineering of the issue.
Copy/paste from that thread:
The chromecast device list in "devices" is truncated to five entries. I have five chromecast audio devices in my home, and using the Google Home app I've created two chromecast streaming groups. The groups contain my devices - for example, my "home" group allows me to play my spotify music on all five devices.
The spotify app on IOS only lists up to five chromecast options, and in my setup, I have seven options: five devices, plus two groups.
Reproducing this issue is easy.
Step 1: Use the google home app on IOS to create chromecast groups until there are more than five chromecast groups + devices available.
Step 2: Using the google music app, select the chromecast streaming button on the top right of the app, note that you see all of your groups + devices listed.
Step 3: In the Spotify IOS app, select the "devices available" button at the bottom of the screen. Note that only five chromecast device + group options are listed.
Screenshots exemplifing my issue follow.
In my home setup I have five chromecast devices: "Janes Room", "Guest Bath", "Living Room", "Master Bedroom", and "Master Bath".
I have two chromecast groups: Group "Home" contains all of my chromecast devices listed above, and group "Not Bedroom" contains all devices except for the "Master Bedroom" and "Master Bath" devices.
In the first screenshot shown below, Spotify shows five items, note that the "Home" chromecast group, and "Living Room" chromecast device are missing from the list.
In the second screenshot taken at the same time, the Google Music app shows all seven items (two groups, and five devices).
After the second screenshot. I removed the "Not bedroom" chromecast group from my setup using the "Google Home" app on my IOS device. I then returned to Spotify's devices list.
The third screenshot shows spotify's device list now shows the "Home" chromecast streaming group, but still "Living Room" is missing.
Note that no matter how many times I restart my IOS device, the devices list remains constant as shown in the third screenshot below.