https://www.theverge.com/circuitbreaker/2017/11/10/16633666/pixel-2-bluetooth-headphones-bug-google-assistant

Google’s Pixel 2 and 2 XL have had a handful of annoying bugs and flaws since launch, and now there seems to be another one preventing the Google Assistant from being used over Bluetooth headphones. Dozens of people have reported the problem on Google’s support forum, and we’ve experienced it as well. It appears to be happening across many different brands of headphones. The problem has been going on since October, but Android Police just called attention to it.