Subtitles not showing when playing a mp4 file

Hi
I’ve noticed that when I add a external subtitle or when there is already a subtitle file with the movie and the file format is mp4, plex says there is no subtitles for this film. I have never had a problem with subtitles with all the others format, with internal or external subtitle that I add myself into the folder. I tried naming the subtitles like the name of the movie file but I had no success. Thanks in advance for your help.

Hi Everyone,

I also experience all subtitles missing on my Shield Android TV. The option to select them is there, even the list of subs is there, but after selecting one it is not appearing.
I use Nvidia Shield to host my plex server, and using the Shield TV client to play content. The subtitles are also not appearing when i am playing content from the same server but with my ipad client.

shield client version: 6.15.2.4143
plex server version: 1.12.4885

Thans for the help.