Yes, I've tried every common troubleshooting step including that. What will fix it briefly is toggling the preferred DNS setting off and on. It will come back until the Spotify client is restarted. That makes me think there is either a bug in Spotify for Android or something weird with WiiM's firmware.
As this is working fine for most people, it's more likely a local configuration issue.
Or annoyingly for some over the internet if it had been previously connected on the local network…
That has never been the case for me.
Oh I've already cleared Spotify, reinstalled. Tried beta/alpha releases, stable etc.
Yes, it very much looks like a Spotify issue. Have you tried to clear the Spotify app cache or even reinstall Spotify?
I still didn't understand what you are referring to as the "cast service". Can you shed some light on this?
Spotify on Android 16 is playing through Spotify Connect flawlessly on my WiiM Amp Pro (not a Pro Plus, of course).
It suddenly resolved itself for me, and I think it's related to the lossless launch because now I have an option with my WiiM endpoint where there will be three dots and it will let me choose between cast and connect. That's the same menu that will be used to also select audio quality.
The fix honestly for me was server side on Spotify's side. It was not a client update. It just happened out of nowhere, which is exactly how lossless will be enabled.
