Android 8.0 has started to roll out to Nexus and Pixel devices, but not everyone is happy with the update. The update has a bug that seems cause the phone to disable mobile data prematurely on some carriers. There’s already an extensive thread on the Google product forums, and a Googler has replied to say a fix is in the works.
This looks to be an issue related to zero-rating data for certain apps and services.
Pixels on Oreo are disabling mobile data on some carriers, Google looking into a fix was written by the awesome team at Android Police.