The Pixel 2 has been in and out of the news here for issues both major and minor, and today we have a somewhat humorous one. Apparently, it is possible for installed apps, like Words With Friends 2, to interfere with the "Now Playing" music recognition feature. Google is aware of the issue, though, and thankfully it's not that big a problem.
The issue was first reported by PiunikaWeb. Based on both the problem's specificity and the low number of reports on the cited Product Forums thread, it sounded unlikely. But, from our own testing, I can verify that the problem, though minor, is real. Rebooting can fix the problem temporarily until Words With Friends 2 is relaunched, and uninstalling the app solves the problem.
So far we don't know of any other apps that can interfere with Now Playing in the same way, but it's certainly possible that others could have the same results.
Fortunately for fans of the game, Google already knows about the problem. In another thread, a representative stated that they're currently working on a fix.
In the meantime, if you're having trouble getting the Now Playing feature in the Pixel 2's Ambient Display working, you might have to choose between the convenience of always-on music recognition and Scrabble-clones.