In latest firmware I no longer can see subtitles in Tizen Browser for HTML5 videos when switching to fullscreen. Others please check if it same issue in your TV and in which firmware it still works correctly.

Examples for test(try both):

HTML5 video+VTT subtitles

https://www.iandevlin.com/html5test/webvtt/html5-video-webvtt-sample.html (shorturl https://goo.gl/sZxQff)

HTML5 video+SRT subtitles

http://www.storiesinflight.com/js_videosub/index.html (shorturl https://goo.gl/0hyAin)