issue with makeMKV files transcoding

[dthree]

This is likely a MPEG2 file ripped from a DVD, as you mentioned. Roku can direct play without transcoding with the following setting enabled.
Check the app settings for "Video codec support" -> "Support Direct Play of MPEG-2 content"
With this off, it will transcode DVD ripped MPEG2 files. Enable it and it will direct play. The bandwidth is higher, so this is not likely to work well remotely, only on LAN.