I have the exact same problem. This is my third Pixel 2, which makes it even more about. I returned my first early on with the random reboot issue, which I believe 8.1 fixes.
With my second, the battery % stopped increasing while charging. It would be plugged in, and show the charging symbol, but the percentage never increased. Very strange.
Now my third has this speakerphone issue, where the speakerphone is on whenever I send or receive calls. It's brutal for answering calls in public. It also happens in the car, whichever is extremely annoying. I can make and receive calls with my handsfree interface, butt all the sound comes from the speakerphone, which is usually in my pocket, making it hard to hear and awkward.