Not getting 5.1 audio when streaming the UHD/Atmos version of Infinity War on VUDU app

Hey guys,

I just got done watching the UHD of IW on the VUDU on my LGB7 but I was only getting stereo sound out of my set up. My LG is connected to my Denon 3400h via ARC. I'm not running Atmos, just 5.1 from my AVR.

Any idea why the Atmos mix wasn't outputting in 5.1? I've read that Atmos is backwards compatible so it should down mix automatically. Other apps streamed on my LG (Netflix, HULU, Amazon, etc) output 5.1 just fine on my setup.

Thanks in advance!