Due to the fact that it is part of Google’s Pixel smartphones, the Pixel XL was one of the first few Android smartphones to receive the Android 9.0 Pie update. Unfortunately it seems that the update came with some problems because according to the Google Issue Tracker (via PhoneArena) website, it seems that some Pixel XL owners have run into quick charger issues following the update.
2016 Pixel XL owners experiencing issues with quick chargers after Android Pie update
Over the past few weeks, some owners of the 2016 Pixel XL have reported problems using quick chargers on Android P. For most of that time, Android P was still in beta, so bugs were to be expected. Unfortunately, issues with quick charging are still present in the final 9.0 release.