This seems to have started in the last day or two. When Ambient Mode kicks in there are new tiles across the bottom that display ads for Youtube videos, podcasts, and some sportsball teams or games of some sort.
Also, instead of rotating through my Google Photos albums, it cycles through these ads for several minutes instead.
Anyone know where to turn off these ads? They are very disruptive.
This turns out to be related to Google Assistant settings.
You have to use the Home App on a phone or tablet. In settings for the Chromecast device, open "Recognition & sharing" and turn Voice Match off.