I totally get the "it's personal between you and Plex" thing. LOL.
Transcoding on the fly is a feature if Plex but it is not the only thing it does. I use something similar to Plex that allows me to turn the transcoding on or off if desired. I can even change device profiles if I have problems. If you are trying to stream a video file that isn't formatted correctly for the device and Plex is either not transcoding it or transcoding it incorrectly then you would get what you have described. I just don't know if Plex has an "on/off" switch that may have been inadvertently flipped or how Plex handles the different device profiles.
Ran across this:
https://support.plex.tv/hc/en-us/articles/200430313-Troubleshooting
Have you tried it with several different file types and gotten the same result; i.e. (.mkv, .m4v, .mp4, .mpg containers with .h264 vs. mpeg-2 video streams or .ac3 vs. .dts vs. .aac audio streams)
Are you not getting any help from the Plex forums or support?
If that doesn't solve it and you are just interested in UPnP/DLNA, you might try Mezzmo. I use that to stream media from my server to my Kodi HTPCs and it has worked nearly flawlessly for several years.