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 to 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. Affected devices produce a notification informing the user that mobile data has run out, and they need to visit their carrier's website. This would be a helpful notification if data had actually run out, but the phone is misinterpreting the situation. Some carriers allow unlimited access to apps like WhatsApp. So, while you might have run through all your data, these apps should still work. However, the Pixel just shuts off data due to this bug. Some users also say the Pixel is shutting off data even when they should have more, which again might be related to zero-rating for certain apps.

Importantly, this is not triggered by the built-in data manager features. It has something to do with carrier features in countries like Honduras, Indonesia, and Romania. Fixing this is apparently a high priority, but there's no solution right now other than flashing back to an older build of Android.