Several Google Pixel XL users are now reporting of a fast charging issue after the arrival of Android 9 Pie stable update. Users complain that fast charging does not work on their Pixel XL after installing the update. One user even reported that a message that reads "Charging Slowly" shows up, even when the unit is plugged into a fast charger. Some report that the original fast charger bundled in with the Pixel XL works fine, but other chargers seem to not be supported anymore.
Google Pixel XL users have taken to the Google Tracking issue site to report this issue. Users complain that fast charging worked fine when the device was running on Android 8.1 Oreo, but ever since the Android 9 Pie software update arrived, this charging issue has surfaced. While some users can still fast charge their phones using the original charger that came bundled with the smartphone, others can't seem to make it work with any fast charger. This clearly seems to be a software issue, as it only seems to have arrived after the Android Pie update. Google is looking into it, and hopefully a fix should come soon. A user reported that a similar issue was reported during the DP4 release as well, but that was closed by Google as "Won't Fix (Infeasible)." This means that a fix for that was not possible, or the AOSP was not a proper place to report the problem. Whatever the case may be, the issue was not resolved in the beta days, and it has trickled into the stable version as well. Note that only Pixel XL users are facing this issue, and other Pixel family devices seem to be unaffected.
Android Pie stable update has currently been rolled out the Pixel devices only; however, third party devices should get it soon. HTC and Sony have already confirmed Android Pie rollout to a slew of devices. Vivo has also confirmed that Android Pie will be rolled out to its smartphones before the end of 2018.