Stereo HomePods output no sound on SD channels

I’ve noticed that my stereo HomePods connected via ARC don’t output any sound for SD channels on the sky box. Also BBC iPlayer has some videos that also don’t output any sound. I think they’re not able to output the stereo sound from older channels, as it’s fine on the HD channels. I changed from bitstream to PCM and can now get sound from the SD channels, but now I don’t have surround sound on Netflix and Apple TV. Is there a way to keep surround sound with bitstream but have sound outputted for SD stereo content?