Vapoursynth HDR to SDR not working as expected

[Selur]

Since you are using HDR->SDR using '--colormatrix bt2020nc ' is wrong, that's a HDR color matrix. Try bt709 instead.
(this would explain, why the preview and the encoded colors differ)

Cu Selur

Ps.: just for completeness Filtering->Vapoursynth->Color->Matrix->TimeCube also has some matrices for HDR to SDR conversion.
PPs.: Also note that NVEnc and FFmpeg->ToneMap both use different default settings for Hable&Co. So it's to be expected that you will different results with the default value.