afaik, yes. From what I have said in the past, yes.
I don’t quite understand why some of the pictures are not coming in…
More on subs and local media assets
https://support.plex.tv/articles/200471133-adding-local-subtitles-to-your-media/
Sadly, I can’t do any testing on my end(NAS RAID failure, rebuilding)
But nevertheless, I would mux .idx/.sub in the mkv… Again, if it was me…