It would seem that the Android Pie upgrade is causing some unexpected hiccups in the 2016 Pixel smartphone lineup. The issue, which has been persisting for some users since the last couple of months, is that the Pixel XL does not seem to be charging rapidly when plugged in even with the standard OEM charger which came in the box.
Though it was believed earlier that it was due to some bug in the Android P Beta program, even after receiving the stable build the problem seems to be still persisting. As per the report by Android Police, the Pixel and Pixel XL support USB Power Delivery (USB-PD) for quick charging and therefore any power outlet certified for USB-PD should rapidly charge the phone. This is demonstrated by the 'Charging rapidly' message on the lock screen.
However, since installing the beta and now the stable build, it would seem that the 'Charging Rapidly' option is not showing up. One of the users on Google Issue Tracker said that "[My] Pixel XL charger says Charging Rapidly every time. All others either say Charging or Charging Slowly — never says Charging Rapidly. This includes 2 Google chargers / USB C cables (1 from 6P pack in and 1 from Google Store), 1 battery backup using a Google USB C supplied cable, and a third party charger and cable. All of which charged the phone "rapidly" 100% of the time before the update."
The page has since been closed down and marked as "Won't Fix (Infeasible)." As per the report the word Infeasible is used "for issues reported that cannot be handled in AOSP, typically because it is related to a customized device or to an external application". There has been no word on Google as to what is causing this problem or whether it can be fixed or not. It seems we will just have to wait and see.