Yeah - I have the same issue. Android Auto Voice control is pretty useless because even if the display sleeps (Android setting and subsequently locks), voice control stops working altogether until I wake it up (which means I have to reach down and grab the phone). The only way voice control works for me for any extended period is to set the sleep time to the max (only 50 minutes) and then, of course, the display remains on.
I would have thought Google would have tested enough to see this as a real safety issue and fixed it accordingly.
What am I missing here?