When you get another chance, can you please submit another log from the app so I can try and trace exactly what is happening? Thx.
I no longer use a Roku device on my main television due to this issue. I using an Amazon Fire TV and it is better but not perfect. Only about 70% of the episodes with subtitles actually play the subtitles. Most of my subtitles are external SRT format and downloaded by Emby. I don't really care if the content is transcoded or direct streamed. I care that it has subtitles. Content is worthless without them.
The web app and the UWP Theater apps show subtitles 100% of the time when the subtitles exist. It doesn't matter if they are in the container or external.