Google pushed the Android Pie update to the Pixel and Pixel XL in the month of August. Immediately after the update the users started complaining that the fast charging had vanished, and now the fix is finally here.
Google acknowledged that the OS update broke fast charging between original Pixel devices and some third-party power adapters that lacked USB Power Delivery, but everything should be back to normal now.
In the patch notes of the build PPR2.181005.003,
Google specifically says that they “Improved fast-charging behavior for Pixel devices” and that the patch only affects the Pixel and Pixel XL.
The October update also includes improvements to Android Auto stability and “improved performance for certain protected media formats” on the Pixel 2. You can download it now by going to Settings —> System —> Advanced —> System update.