Over the past couple of months, an thread began over at XDA-Developers regarding an issue with the Google Pixel XL no longer "Charging rapidly" when plugged in, even with the factory OEM charger. This issue was occurring to those who installed the Android P Beta. But even after Android Pie's final release, there is still no resolution for some folks. The problem seems to be affecting a number of 2016 Google Pixel and Pixel XL owners who report they cannot get the phone to "Charge rapidly", with either the factory charger, a USB-PD compatible power bank, or even an aftermarket USB-PD...
Source: https://www.gsmarena.com/android_pie_breaks_fastcharging_for_some_2016_pixels-news-32743.php
