I have a Google Home display and just purchased a Nest Hub display. These devices were added to a speaker group but when casting to the speaker group, neither display played music. I was able to fix this by disabling Ultrasound Sensing and Personal Results on both displays in the Google Home app. I think the Ultrasound Sensing is what fixed it. Regardless, I'm just happy I can cast to both devices in my speaker group. I hope this helps someone else.
