I’m not sure if this is an LG webOS issue or not but while I’m playing an episode with an external SRT subtitle, it plays the subtitle just fine until after a few minutes it just suddenly disappears. I also have some episodes in my Android mobile Plex app where the external SRT file is selected but there is no subtitle when I play them.
Here are my PMS logs: https://www.dropbox.com/s/vdnlf19mgbzsobu/PMS%20Logs.zip?dl=0
My PMS is on Version 1.19.1.2645 and a sample media that I played earlier which had the problem on my LG webOS was:
The Neighborhood S01E09