Volumio does not show up when querying Spotify API for available devices - this is among other things used in integration scenarios with e.g. Home Assistant - and essentially this makes it hard/impossible to use Spotty with Volumio in those scenarios.
Create the API Client ID in your Spotify account and get OAuth 2.0 token from https://accounts.spotify.com/authorize in e.g. Postman to reproduce this bug.
Auth Settings in Postman:
Bearer
Implicit Grant Type
Scope: user-read-playback-state
When exercising the API call Volumio is not returned a Connect Endpoint available to Spotify.
Discovery I guess - there are two spotify plugins, the one from the Volumio Team where you log in with your credentials and the one I’m trying to use to get Spotify Connect from apps and other systems.
Spotify Connect will not show up in your devices if you don’t log into it. In Discovery mode, well it needs to be discovered first before it shows up as an active device
Spotify Connect with the Discovery feature, uses mdns and other trickery to show a device as available from other clients inside the local LAN.
Such a device will not show up under the API, unless it is in active use. It will then show up under the respective user’s devices as an available/active device.
So I don’t understand why you’d expect it to show up in the Get a User's Available Devices endpoint, because – well, it isn’t a specific user’s device when still in discovery mode.
If you want the device to be locked to once account, then go to the plugin settings page, and enter your credentials. It will then show up in the /me/player/devices endpoint.