I've had more chance to play today and can reproduce this 100% of the time.
The controller has been successfully linked with the Chromecast. However, each time I start up a game, within 30-90 seconds, the game reports that the controller is disconnected. However, I can still access the Stadia overlay with the controller. To fix it, I have to shut down and relink the controller reach time I start up a game
Same issue here, any news? My WiFi signal is good and chromecast is wired connected. Home is fiber connected (800mb/s). How can I connect Stadia support, I think the controller is the issue since some people don’t have the issue. Thanks!