TLDR: Until MS allows third party apps to fully utilize bitstream like the bluray app does, PCM is the way to go for plex.
I think what is going on is that Xbox let's you choose to bitstream from the blu Ray player app. You don't need to pick Dolby or DTS. It seems like Microsoft put this functionality in for its player so it would be compatible for atmos etc and meet the bluray spec. I don't think third party apps like plex have this full capability yet. So if you want to bitstream you need to pick Dolby or dts which will force the box to transcode the audio if in a different format.