Code
08:19:25.542 T:139695467251456 DEBUG: CDVDVideoCodecFFmpeg - Updated codec: ff-h264-vdpau
08:19:25.545 T:139695467251456 DEBUG: CVDPAU::SetWidthHeight Setting OutWidth: 1920 OutHeight: 1080
08:19:25.545 T:139695467251456 NOTICE: (VDPAU) screenWidth:1920 vidWidth:3840 surfaceWidth:3840
08:19:25.545 T:139695467251456 NOTICE: (VDPAU) screenHeight:1080 vidHeight:2160 surfaceHeight:2160
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Getting back to the OP, the log shows that the VDPAU code initializes the downscaler early in SetWidthHeight() and sets the output screen size to the current screen size of 1920x1080. As this is part of the VDPAU Mixer the behaviour can be disabled with the Mixer setting.
Just my five cents, I know that this does not help you.