I have found that, when I try to show external SRT subtitles while playing an MKV file which has embedded ASS subtitles, selecting the external SRT to be shown, the client just shows the embedded ASS subs, The same occurs when I try to show OpenSubtitles with this mkv file.
This issue is just related with the Fire TV client (both on my Fire TV 4k and on my Fire TV Cube), as the behaviour on iOS, tvOS, web clients is OK, I mean, it shows the selected subtitles.
Server Version#: 1.23.0.4497-600 (the latest beta on Synology NAS)
Player Version#: 8.17.0.25120 (f80c7b2d) (the current firetv version)
I have the same problem wit STR subtitles on my android tablet and Sony tv.
The only when it works if I use my Iphone and chromecast the content to my Sony tv