Setup: Xbox One X wired with CAT6 1GB and HDMI to LG C9. Streaming from a Windows 10 PC over 1GB CAT6 with Core i7-6700 with 16Gb ram and SSDs.
Problem: Large MKV files (anything above 13Gb or so) cause Plex player to buffer when an attempt to fast forward or rewind and then freeze at the buffer. The same files do not cause Microsoft media player on the Xbox to buffer. Fast forward and rewind works ok there. The same can be said the the plex app on an LG TV, fast forward and rewind works there too.
Testing: I set the Xbox One X player to transcode the files, and it still freezes on fast forward or rewind on large mkv files.