ffmpeg – Why is the scale filter ignoring the `out_color_matrix` option?

I’m testing on an input that has none of its colour metadata set:

ffprobe -v error -show_streams "input.mp4" | grep color

color_range=unknown
color_space=unknown
color_transfer=unknown
color_primaries=unknown

This results in a colour shift, which I believe is because of the automatic colorspace conversion that the scale filter does. However, when I add out_color_matrix=bt601 to the scale filter to preserve the colourspace, the option seems to be ignored entirely and the colourspace is still converted to BT.709 (metadata is written because I have -color_range 1 -color_trc 1 -color_primaries 1):

ffprobe -v error -show_streams "out_color_matrix=bt601.mp4" | grep color
color_range=tv
color_space=bt709
color_transfer=bt709
color_primaries=bt709

Why is this?