One of the annoying things I've noticed with the new Chromecast is when watching Twitch, every 15 minutes ambient Mode starts up. This is pretty annoying and I've never seen that on my Shield. It's annoying enough that I will be using my Shield TVs until it's fixed. AFAIK, you can't even configure the Ambient Mode start time either.
Samething is happening on my end, only on Twitch. It's pretty annoying. I've also turned off energy optimization for Twitch and it still does it.
I hope it gets fixed soon. I submitted feedback.