There are quite a few posts appearing about this problem, i also now have it on my LG tv. For some reason subtitles seem to be ON by default instead of OFF, and there's no main setting to override this so you have to turn them off each time you start watching something on the app. Do you know who to tag about this so hopefully it will be fixed??
if I remember correctly. While subtitles shouldn't be switched on by default the app will remember what the subtitle state was when you last used it so if you had subtitles switched on when you last used the app and didn't turn them off before exiting playback, then the next time you use the app assuming that subtitles are available for the piece of content that you are watching. Subtitles will be switched on when you place that content.
With NowTV hardware/Roku things work a bit differently because there is and 'always on' setting that can be used for subtitles.