The exact same thing is happening to me. The code I used is the same as the one you've provided, and it worked with firmare 8.0. Unfortunately, I can't find any specific information about this specific firmware change, nor any concrete examples on the official Roku documentation.
Perhaps I've been doing things wrong all along, and it worked just out of coincidence, and with firmware 8.1 something might have been deprecated that finally broke my code. However, there's no error message or any kind of signal that may indicate what I am doing wrong.
I will try setting the subtitles URL on the ContentNode object, as suggested. However, in my Channel Application, the subtitles track URL may change at any time during playback. In such case, I don't know if a change in the ContentNode object will be detected or applied by the SceneGraph Video node once the playback of the video has started. Would that work?
If you find a solution, please share it. I will do so, too.