I have the "Stadia Premiere Edition" which consists of a Chromecast Ultra and a controller. (so I am not "casting" from another device, it's playing direct off the Chromecast Ultra. The first time I plugged it in and turned it on, I was able to play games in 5.1 surround sound. The next time I tried to play the same game which was is 5.1, it was only coming in at 2.0 stereo. I found that if I rebooted the Chromecast, the 5.1 surround sound would come back. This happens every single time....so any time I want to use it I have to reboot it right before playing Stadia or 5.1 does not work. I have tried a factory reset of the Chromecast and it did not solve the issue.
Having just rebooted the chromecast to regain 5.1 rather than stereo, it is as though the chromecast doesn’t inform the amp of the 5.1. After reboot it starts of in Stereo, then I launch Metro Redux you see the amp change to 5.1 as it gets the signal.
Somehow this isn’t being passed after the first time, could just be a Denon issue.
I first tried Stadia and it started in Stereo. I then tried Netflix and it switched to Dolby Digital +. I then switched back to Stadia and then it changed to "Multi In" and the surround sound worked. So I'm not sure if this helps or not....still can't determine if this is a Chromecast issue or a Stadia issue. I'm also not sure the difference between DD+ and Multi In as both appear to be 5.1, but different versions of it.
Did you manage to fix this I have exactly the same issue, is your receiver a Denon? I have a X2300W.
I have to reboot every time like you say to get the 5.1 back instead of 2.0