After Google started to roll out Android 9 Pie update, reports started to pop up claiming that updating to Android 9 Pie breaks Google Pixel's and Pixel XL's fast charging, and it seems like Google is working on a way to fix that. According to a statement that Google shared with The Verge, the company is well aware that "non-Power Delivery (PD) USB-C chargers no longer rapidly charge the 2016 Pixel and Pixel XL after upgrade to Android 9 Pie".
The company went on to say that the 18W rapid charger, which is included in the box with the Pixel and Pixel XL, is a PD charger, and is not faced with such issues, but the company is working on a way to amend the issue for the non-PD USB-C chargers. In addition to that, the company has promised that a fix will be rolled out in the coming weeks, so those of you who are having such issues will have to wait a bit longer. So, in case you're a bit confused by all this, it seems like only third-party charging plugs are having issues with fast charging the Pixel and Pixel XL, while the original charger should work just fine even after the update, and other chargers that are certified for USB Power Delivery.
Reports regarding this issue started surfacing soon after the company started pushing out Android 9 Pie, and it seems like quite a few people are using third-party fast chargers with the Pixel and Pixel XL, as a ton of reports surfaced online, mostly on Reddit. Google quickly realized that the issue did not come from their own charger, but from third-party offerings, though it seems like it took the company a bit of time to fully realize what's the issue. All in all, if you're having such issues, and you still own a fully functioning original charger that came with the Pixel / Pixel XL, use that for the time being.