MKV files with embedded subtitles

I run Plex Media Server on a Synology NAS and have a Samsung Smart TV with PLEX installed. When I run mkv files with external subtitles this works fine. I always thought that Plex does not support embedded subtitles but now I read on several sites that Plex is also supporting mkv files with embedded subtitles. But I cannot get it working. I have the latest version of all software installed. How can I get this working ? Please be specific, because I am not an experienced user

sorry … mov files should be mkv files

It also makes a difference which format these embedded subtitles are in.
And the particular model of your Samsung TV is also important.

If you have a SmartHub model which runs Orca’s app https://forums.plex.tv/categories/samsung-smart-hub then it only works with ‘sidecar’ subtitles in the SRT format.
Everything else may either run poorly or may require transcoding, which your NAS is unable to do.
https://forums.plex.tv/discussion/66729/orcas-mkv-fix-tool-version-1-2-new-update/p1#top

I also have a problem viewing mkv files with embedded subtitles. It was working fine in the past and stopped working. The files are working fine in ios app and in the browser but no subtitles when you watch in the Samsung tv app (I have 55es8000 tv)