I have same issue with an LG 65QNED993PB.
I checked this topic To Enable Subtitles and Transcode Unsupported Audio also Forces to Transcode Supported Video and @FordGuy61 suggesstion (use a compatible audio track) is solved the continuous buffering.
But In my case the subtitle was a UTF-8 encoded normal text based SRT was embedded in the MKV container…
So not just PGS or VOBSUB subs trigger this video stream renecode. 
