Probably a software issue - the TV is reading the incoming SDR signal and treating it as Rec. 709 (the SDR standard) as opposed to Rec. 2020 (the HDR standard). Since the color primaries for Rec. 2020 are much wider, the values in the signal representing the "amount" of each color are smaller. If you then treat those numbers as in the SDR gamut, you get a washed out (less saturated) image.

This is also just a shot in the dark, but matches your observations pretty well.