Plex on Tivo only sending PCM

I have recently started using Plex. I have the Plex Server (1.13.0.5023-31d3c0c65) running on my ASUS NAS. I'm using the Plex client on my Tivo (Series 5 on software version 21.8.1.RC6).

I've used Handbrake to produce MVK files onto the NAS. They have AAC or AC3 5.1 audio tracks and when I display their information on the Plex client it reflects the 5.1 audio track.

However, when I play the movie through Plex and run the audio through my Onkyo receiver the Onkyo shows the audio track as PCM with 2 tracks.

Is there a reason that the Onkyo wouldn't receive a full 5.1 audio track? Thanks!