This relates to Windows resolution settings. When a new resolution is selected, the signal will be output in native resolution and then scaled down to the resolution you choose, leading to a compressed or stretched image.
Below are the solutions:
Dedicated Nvidia/AMD GPU: Go to Display Settings -> Advanced Display Settings -> Display Adapter Properties -> List All Modes, choose a lower resolution, apply it, and then switch back to the resolution for PBP 5:9 again. You will get the correct aspect ratio now.
Intel Integrated GPU: Open the Intel Graphics Control Panel -> Scaling -> Choose "Maintain Display Scaling" -> Click Apply. You will get the correct aspect ratio now.
* To achieve PBP 5:9 when running a single system, make sure that your system supports two video outputs.