We talked about this offline, but I figured I'd throw my two cents in here:
My Google Play Movies Android app works fine on my Pixelbook -- but this was after I first uninstalled it, and then reinstalled it to see if I could duplicate Mike's problem. Since it does work normally on mine, I wonder if it had to do with uninstalling the version that came with the Pixelbook. Or maybe I'm just special.
Another thing is to make sure Android System Webview is up to date.
In any case, I find that the Chrome app version of Play Movies works just fine (and is more optimized for the Chromebook screen). That's why I uninstalled the Android app pretty soon after I got the Pixelbook (along with the other Google Play Android apps like Play Books and Play Music).