This problem has been an issue for awhile. All videos with TrueHD audio appear to cause a memory leak with the transcoder and fail. See this thread: https://forums.plex.tv/t/optimizing-4k-titles-causes-plex-to-freeze-truehd-audio-the-problem/238970/17
It seems there may finally be some momentum for getting a fix, as discussed here: https://forums.plex.tv/t/conversion-failed-the-transcoder-exited-due-to-an-error-memory-allocation-failures/274811/53
