Hi guys,
There is a problem about 2.5 years long that on some mkv files subtitles were not showing. Very weird bug, because it was depending on file location, if file, for example, was on e: potplayer didn't show subtitles, but if the same file was located to d: disc, potplayer did show subtitles.
I saw a lot of topics with this problem, but there was not working solution in there. I used an old player version for a while, but someday I became too desperate and tried to find the reason caused such behavior. And I found it! The "SeamlessPlay=1" setting in config file, which seems to be obsolete, caused subtitles not showing. If you delete this line, subtitles will work fine.

I have already published this on PotPlayer forum, but beacuse your forum is better indexed by Google, I duplicate it here for some people to find it useful.