The transcoding to MP3 would happen to me with AC3\EAC3 5.1 converting to MP3 stereo on my Roku Ultra (and TCL) with MKV files. Others are getting 7.1 converting to MP3 stereo instead of 5.1 - 5.1 to stereo transcoding was able to be worked around with some settings changes (but then triggers a Roku AC3 audio delay bug as well which required additional tweaks). There’s another linked thread with more technical info in it earlier in this topic (see my previous post) that has more references (transcoding logs) and goes over that AC3 specific bug as well which might help you with any testing.
Edit: Here’s a link directly to my post explaining the two different bugs impacting Roku from the other thread: Roku and Chromecast transcoding AC3 5.1 to MP3 - #35 by Insomnic_1
