Non-Atmos audio issues/no sound when other TV input is still on

Having a weird audio issue with my new ATV 4k (3rd gen). I get no audio when I stream content that’s NOT Atmos, so usually movies/shows with 5.1 etc. This only happens though when I have the cable box still on in another TV input (cable box audio is some sort of Dolby Digital via HDMI). But if the cable box is OFF, or I change its audio setting from Bitstream to PCM, then the ATV works fine for all content.

When I play the non-Atmos content without sound on ATV4k, the TV doesn’t display an audio type (eg it doesn’t say “PCM” or “DD” anything). But if I turn the cable box off at the same time, the PCM label suddenly appears and the audio on ATV content works again.

Atmos content plays fine always. Firestick and native apps do not have this issue. I’m thinking this has something to do with the way ATV outputs audio differently, but I can’t figure out why this would conflict.

HDMI path

ATV (and others) —> LG C2 TV —> LG SP9YA soundbar via eARC.

ATV audio settings: Change Format off, Auto/Dolby Atmos

TV: Bitstream set, eARC, I’ve tried both Auto and Passthrough and no difference

HDMI cables are up to date

I’m aware of the workarounds, but all seem to be compromises or take on other risks… plugging directly into the soundbar works but switching inputs is not easy, and if I have sync issues I’m giving up. And I’d prefer to not have to go basic stereo sound for the cable box. Any idea what I can try to fix this? Thanks!