Hey. Did you get screen casting to work? What is your setup?
I was never able to get casting to work at all, but I've only tested on a Roku. Don't have any other device at my disposal.
But, yeah, I noticed this too in a different way. Same behavior if you were to "Pop-out" the video, the subtitles stay in it's original location and no subs show up on the pop up video.
What a friend of mine did was to mirror his Samsung phone screen to his Samsung TV, and that worked. It may not be ideal, but could you try that and report back as well?