My chromecast panes start off with “on/off” icon (the zero with a one inside) which I can click to turn the device on. However, after I do that, the icon switches to a play/pause button, and there seems to be no way to turn the device off. The only way I’ve found to turn it off is to launch the Google Home app on my phone and turn it off from there. I’m nearly certain that it used to be possible to turn the chromecast off from the GUI, as I think I remember doing that.
It looks to be a change to the default more-info popup, but the custom card Mini Media Player still works the way you’re expecting.