As of Spotify 8.9.xx and newer I can't control the volume on my iPhone when casting to a Chromecast device. I need to go into the slider inside the Spotify app to change the volume.

This is not an iOS or apple issue:

My non working version of Spotify is 8.9.16.556 and just today updated to 8.9.18.xx which has the same bug.

On another iPhone a WORKING version of Spotify is 8.9.14.526

The latter works perfectly fine. It is not a iOS problem but a Spotify bug!

Both iPhones run iOS 17.3.1

If this is not being actively solved by Spotify I'm sorry to say I will cancel my subscription and change to Tidal. This is a CORE functionality.