[BUG] Vapoursynth: Image Stream Luma TV Range not working correctly (??)

[Selur]

Except, the output video file is at "PC" range instead of "TV" range. Shoudn't the x264 output range change to "TV" if "Range Conversion" -> "full to limited (pc -> tv)" is specified on the "Color" tab for Vapoursynth?

Yes, the video would be tv range, the same as the source images.
If you want to produce TV range out of PC range you should use the Color Matrix filters.

# Setting color range to PC (full) range.
clip = core.std.SetFrameProp(clip=clip, prop="_ColorRange", intval=0)
clip = core.resize.Point(clip, range_in_s="full", range_s="limited")
# Setting color range to TV (limited) range.
clip = core.std.SetFrameProp(clip=clip, prop="_ColorRange", intval=1)

this does a PC -> TV range conversion, which is okay, but your VUI signaling does not match your content your playback thus the colors in your output will be 'off'.
If you use the Levels filter this does the same.

If you use the Range Conversion to TV scale or change the luma levels using make sure you signal TV scale and not PC scale in your encoding settings, otherwise your output will be 'off'. My guess this is what causes part of the confusion. (In some cases Hybrid tries to adjust the encoding settings, but not in all, since it can't be sure what is done why. )

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I can't say what's happening when you use Avisynth, since you did not share the used script and encoding settings.

Cu Selur