Oreo bug incorrectly shuts down mobile data on Pixel phones used on certain carriers1
The devices involved are sending out notifications to users telling them that their data allowance has run out. But that is not true. It appears that the phones are misreading zero-rated data. For example, let's say that your carrier allows unlimited service to WhatsApp. Even if you've burned through all of your monthly data, you still should be able to access WhatsApp. However, because of the bug, the Pixel will just turn off mobile data. This also happens in other situations where the Pixel user has more data to use under his/her monthly allowance, and for some reason mobile data is still shutting down.
The word around the water cooler is that this is related to the carriers in markets like Honduras, Indonesia, and Romania. And while Google says that this is a top priority, right now the only solution is to flash back to Android Nougat.
source: GoogleProductForums via AndroidPolice