@jbsjbs, unfortunately I don't think there is a super simple cookbook way of "fixing" an MP4 file so it won't have this problem.
Here are some pointers, which is a slightly more expanded version of my reply to @VanillaDrizzle. Keep in mind that the problem happens because the Plex server finds it necessary to transcode and/or remultiplex the file for some reason. This is often because of the AAC audio. Plex switches to an MKV container and correctly puts color space information into the header of the container. The color information in the header triggers the Roku Bug. The strategy is to convert them so that Plex doesn't try to transcode or remux them. They can then can be direct played from a container that doesn't contain the problematic color space info in the header. Here are the steps I use:
1. Use FFMPEG to copy the existing audio and video streams to a new MKV container. At the same time transcode the existing AAC audio to and AC3 stream, and add that to the container as well. Note that FFMPEG is a VERY powerful command line utility that has a somewhat intimidating learning curve. There is a great GUI called myFFmpeg4 (costs a few bucks) that makes it much more approachable.
2. Use a tool such as MKVToolNixGUI to remove all the colorspace and color mastering information from the header.
3. Put the new MKV file in your Plex library in place of the old one. Make sure your Plex server and client are set up to direct play the file.
This has been working well for me. It would be better (sigh) if Roku would just fix the bug though.
