In a statement provided to Android Police, Google said, “We’re aware of an issue where non-Power Delivery (PD) USB-C chargers no longer rapidly charge the 2016 Pixel and Pixel XL after the upgrade to Android 9 Pie. The 18W rapid charger included in-box is a PD charger and does not exhibit this behavior. We are verifying a fix for non-PD USB-C chargers and will roll it out in the coming weeks.”
It is unclear as to what might have caused the problem, but it doesn’t seem to be brand-related as users are reporting issues with different brands of chargers. Also it appears that it does not appear to affect only the Pixel XL as there are 2016 Pixel owners who are also reporting that they have run into the problem themselves, but for the most part it appears that the majority of complaints are coming from Pixel XL owners.