It appears that Google has messed up how its Pixel smartphones how they handle notifications when set to Do Not Disturb (DND). Originally reported on Reddit, some Pixel devices are receiving two notification sounds too. Google cannot seemingly fix the issue either.
https://www.notebookcheck.net/Some-Google-Pixel-smartphones-may-have-a-Bluetooth-related-DND-notification-bug.422966.0.html
I'm pretty sure this exact thing was happening on my OnePlus 6T 2 days ago and I also noticed that using Bluetooth was triggering it. I'm not sure it's exclusive to the Pixel.
I a have a Pixel 3 XL with Q 3 beta. I have non of the issues mentioned so I can assume those bugs will be fixed in the next OS update to Android Q.