I have this same issue
I'm loading external subtitles (SRT format, without any overwrite) to a nexus players, and seems that it's keep the original black and small size.
try to relauch, recast to reaload app, but still don't work.
I appreciate some help, otherwise I do see any advantage in premium features.
Edit: test on a 1 gen Chromecast and works perfect, it's seems some problem with nexus player and yours google cast support
Edit 2: Just realize that mandatory settings for subtitle are made in google play movie settings. It will overwrite videostream settings