That's the thread I started in r/PleX for background. I was encouraged to make a post here to continue looking into the situation.
1) Is the Roku Ultra transcoding audio? PMS insists it is in directplay mode yet DTS-HD tracks are being converted to DD+ somewhere.
2) Why are some of my DTS-HD tracks being played back properly (the core only), which, according to LJunkie, is unexpected until Roku fixes the bug.
I enabled debug and remote logging and played back two files - one plays the DTS core, the other transcodes to DD+. Time was 8:22-8:24PM USA central time.