I’ve been having the same issue. I bought an Ultra–the second version (4660X running 8.0.1 build 4047-46)–back in December. I had upgraded my home theater, and my old Roku 3 was not parsing the AAC 5.1, and also could not do 4K. After researching on the internet, I was lead to believe the Ultra would pass AAC 5.1 to my receiver–whether it be the first or second version of the Ultra. And, for about two months, it WAS passing AAC 5.1 to my receiver (Marantz NR1608), to be decoded as DD 5.1. All was good. About 3 weeks ago, it started only passing 2.0 PCM. As far as I could tell, there had not been any updates to the Roku firmware, receiver firmware, or the Roku App, but something definitely changed. After 3 weeks of frustrating nothingness from Roku support, I actually bought another Roku Ultra today, thinking it was simply a bad Roku–nope–same issue with the new one. So, I went back on-line, again, to search for answers, and came across this thread. Anyway, I just downloaded the updated Plex channel linked above by ljunkie, and it worked immediately–my AAC 5.1 content once again resolves as 5.1 DD on my receiver–yea!
Big props to Plex for acknowledging and fixing this issue so quickly. This is why I recommend Plex to everyone I know, and also why I upgraded to Plex Pass, even though I don’t use any of the advanced features–I just felt it was necessary to support Plex for their unique and robust product. I used to also recommend Roku to everyone, but now I would think twice about that. I mean, really, my 3-year old Chromecast that I pulled out of my junk drawer was able to resolve the AAC audio to 5.1 (or at least pass it to my receiver to resolve), and my brand new Roku Ultra could not–ridiculous!
Edit: In case it helps either Roku or Plex figure out when this changed, I called Roku support on 2/9, and my system was working 1 or 2 days before that, so it stopped working about 2/7-2/9.