You have to look for the MDE: entries in the Plex Media Server.log files. They will tell you exactly why your media was transcoded and why Direct Play or Direct Stream was not an option (because that is what’s happening here).
Attached is the last example from your Android client. You have Direct Play disabled in that client and the file has a much too high bit rate ^^ 10 Gbps. That’s surprising ^^ Either the file is broken or PMS does some calculations wrong on the Asus Tinker Board.
Btw: Please do not use this board for PMS. It’s clearly designed to be an Android developer board and the CPU and board have problems with Linux. The overall experience was not very great here in the forum.
Jul 08, 2018 11:17:45.537 [0xad3ff400] DEBUG - Streaming Resource: Calculated bandwidth of 10500000kbps exceeds bandwidth limit. Changing decision parameters provided by client to fit bandwidth limit of 20000kbps
Jul 08, 2018 11:17:45.535 [0xad3ff400] DEBUG - Audio Stream: 7109, Subtitle Stream: -1
Jul 08, 2018 11:17:45.536 [0xad3ff400] DEBUG - MDE: Selected protocol hls; container: mpegts
Jul 08, 2018 11:17:45.537 [0xad3ff400] DEBUG - MDE: analyzing media item 2956
Jul 08, 2018 11:17:45.537 [0xad3ff400] DEBUG - MDE: E1 - Chapter 14: Direct Play is disabled
Jul 08, 2018 11:17:45.537 [0xad3ff400] DEBUG - MDE: E1 - Chapter 14: media must be transcoded in order to use the hls protocol
Jul 08, 2018 11:17:45.537 [0xad3ff400] DEBUG - MDE: <redacted> - S2 E1 - Chapter 14: selected media 0 / 2956
Jul 08, 2018 11:17:45.537 [0xad3ff400] DEBUG - Streaming Resource: Calculated bandwidth of 10500000kbps exceeds bandwidth limit. Changing decision parameters provided by client to fit bandwidth limit of 20000kbps
Jul 08, 2018 11:17:45.537 [0xad3ff400] DEBUG - Streaming Resource: Determining preferred transcode encoders through transcode only decision.
Jul 08, 2018 11:17:45.538 [0xad3ff400] DEBUG - Scaled up video bitrate to 774Kbps based on 1.500000x fudge factor.
Jul 08, 2018 11:17:45.542 [0xad3ff400] DEBUG - Scaled up video bitrate to 774Kbps based on 1.500000x fudge factor.
Jul 08, 2018 11:17:45.542 [0xad3ff400] DEBUG - Streaming Resource: Reducing playback quality for 19047kbps stream bitrate: video resolution to 704x352, audio channels to 2, quality to 99, disable video DS as 2147483647kbps is > the 2478kbps available, disable audio DS as 2147483647kbps is > the 256kbps available
Jul 08, 2018 11:17:45.542 [0xad3ff400] DEBUG - Scaled up video bitrate to 774Kbps based on 1.500000x fudge factor.
Jul 08, 2018 11:17:45.543 [0xad3ff400] DEBUG - MDE: Selected protocol hls; container: mpegts
Jul 08, 2018 11:17:45.543 [0xad3ff400] DEBUG - MDE: analyzing media item 2956
Jul 08, 2018 11:17:45.543 [0xad3ff400] DEBUG - MDE: E1 - Chapter 14: Direct Play is disabled
Jul 08, 2018 11:17:45.543 [0xad3ff400] DEBUG - MDE: E1 - Chapter 14: media must be transcoded in order to use the hls protocol
Jul 08, 2018 11:17:45.543 [0xad3ff400] DEBUG - MDE: E1 - Chapter 14: Direct Streaming is disabled, so video stream will be transcoded
Jul 08, 2018 11:17:45.543 [0xad3ff400] DEBUG - MDE: E1 - Chapter 14: no remuxable profile found, so video stream will be transcoded
Jul 08, 2018 11:17:45.543 [0xad3ff400] DEBUG - MDE: Cannot direct stream video stream due to profile or setting limitations
Jul 08, 2018 11:17:45.543 [0xad3ff400] DEBUG - Scaled up video bitrate to 774Kbps based on 1.500000x fudge factor.
Jul 08, 2018 11:17:45.543 [0xad3ff400] DEBUG - MDE: E1 - Chapter 14: Audio Direct Streaming is disabled, so video's audio stream will be transcoded
Jul 08, 2018 11:17:45.543 [0xad3ff400] DEBUG - MDE: Cannot direct stream audio stream due to profile or setting limitations
Jul 08, 2018 11:17:45.543 [0xad3ff400] DEBUG - MDE: <redacted> - S2 E1 - Chapter 14: selected media 0 / 2956
ul 08, 2018 11:17:45.544 [0xad3ff400] DEBUG - Streaming Resource: Session 0xb0675f00:5d62777be92118e1-com-plexapp-android changed transcode slot usage to used. Used slots is now 1
Jul 08, 2018 11:17:45.544 [0xad3ff400] DEBUG - Streaming Resource: Reached Decision id=1218 codes=(General=1001,Direct play not available; Conversion OK. Direct Play=3000,App cannot direct play this item. Direct play is disabled. Transcode=1001,Direct play not available; Conversion OK.) media=(id=2956 part=(id=2956 decision=transcode container=mpegts protocol=hls streams=(Video=(id=7108 decision=transcode bitrate=774 encoder=libx264 width=704 height=352) Audio=(id=7109 decision=transcode bitrate=256 encoder=aac channels=2 rate=48000))))