Hello,
I am experiencing what seems to be the same issue. I am using the Spotify Connect 2 plugin.
If I have a podcast playing in the official Spotify app, the Volumio device shows “Unavailable to control” under the “Connect to a device” pane. The workaround is to start playing a song, connect Spotify to the Volumio device, then switch to the podcast.
Volumio Version: 2.909
Volumio Spotify Connect 2 Version: 1.0.5
Hardware: Raspberry Pi 4 Model B