Hey Costa I wonder if there might be something else at play here. I have many Anime shows with embedded ASS subtitles and external SRT subtitles (as an option for compatibility reasons). I tried a few episodes just now and was able to seamlessly switch between both the embedded ASS and external SRT subs on my 4k Firestick. Any possiblity it might be a permissions issue (has definitely never happened to me before 
)?
Edit: Also you mentioned having multiple subs. Any chance there are 10 or more subs for that file? If so you me be experiencing the same issue as me here. Plex for Fire TV seems to be bugged at the moment regarding files with 10+ subtitles
