Upon receiving my Pixel XL, I setup the data usage montior and set it to notify me at 1.5GB. After about 10 days, it altered me that I had used that much mobile data, which is extremely unusual. So, I started looking at which apps were using data. The main culprit was Android OS. Digging a bit deeper it had used 1.3 GB of background data! This is data that an app uses without you knowing it!
After a bit of Googling, I tried to turn off automatic updates and turn on data saver (shutting all apps off). Over the course of the next 4 hours, the background data grew from 1.3GB to 1.4GB. I had to turn off mobile data altogether to get it to stop.
That night, I preformed a factory reset and downloaded the latest firmware update (I think I was still on the Dec update) per Google research. Since then, I have a strict data monitor setup and everything seems to be normal.
My question for the community, should I be responsible for the data that was used by the OS in the background that I had no way to control? I feel like I had to go to the extreme (factory reset) to resolve the issue. And if I hadn't setup the data monitor, I'm sure I would be hit with a data overage before anyone noticed.