Just signed up to Stadia and I am experiencing the same issue. I am on MacBook Pro (13-inch, 2019) using Chrome Version 80.0.3987.163 (Official Build) (64-bit). I'll play a round or two and I will loose control of either steering or throttle/brake. I've bound to the arrows keys. The keys will work fine in the in-game menu, but not in race. It will work after I've exited the game and restarted.
Worth mentioning is, that it seems to register the press of the key, as in when you loose control of reverse, it will show R and a quick response when just pressing the button. But it doesn't respond to holding down the key.