C9 eARC not passing TrueHD+Atmos through to receiver
Using the two Nature's Fury files from https://thedigitaltheater.com/dolby-trailers/
If I play through DLNA or directly from either Plex or Emby a DD5.1+Atmos file, Atmos is fine. If I play the TrueHD+Atmos version the TV reports it as not compatible so it forces transcode.
I figured I'd try directly playing the files since the TV supports MKV so I put them on a USB drive. The DD5.1 one doesn't show any errors but also doesn't actually output any sound. The TrueHD one pops up an error saying that no sound is available for the file.
There appears to be some specific Denon/C9 issue that seems squarely in LG's camp: https://www.avsforum.com/forum/40-oled-technology-flat-panels-general/3072900-lg-c9-earc-info-thread.html but given that the TV literally says it can't decode the file from USB I'm not sure if it's the same issue.
I also have heard something about cables or cable distances mattering. Is there any way to know whether the C9 thinks that it is using eARC successfully so if there's a cabling issue I can track it down?
This is my first new TV in 11 years and one of the reasons I held out for the C9 was forwards compatibility with HDMI 2.1. It's possible this is all coming in future firmware updates but information on that is pretty scarce. Does anyone have suggestions on what's going on here?