I was running 1.9.6, after an auto update to 1.10.something subtitles stopped working on Chromecast. I looked around and found a thread[1] with many people reporting the issue with many versions of Plex. One guy said he reverted to have it working again so I tried that and... it worked! I haven't updated Plex since then. Good to know that you have subtitles working with the latest version, maybe I will update when I get home, fingers crossed!

[1]https://forums.plex.tv/discussion/288806/plex-losing-subtitl...