I'm running Libreelec 9.0.2, had the same problem with 9.0.1 as well, on a Intel NUC D54250WYK and I'm having trouble that my external subtitles are not shown automatically when I'm starting a movie or tv show. The files are played over a NFS-share and the subtitles are named as the video files with .sv as language tag, and all subtitles are in srt-format. The subtitles are loaded correctly since I can start them at once when the video is running. And it's the same for MKV-files that got embedded subs as well.
In the settings I've set the audio language to original and to prefer standard audiostreams. And as the prefered subtitle language I've set it to Swedish, but it's not loading anyway. Anyone else who've got the same problem?