When subtitles are inside a matroska file, more often than not, they won't work.
Same problem with multiple Audio too

The problem is only with BSPlayer as VLC and MPC they do work
It also were the case that BSPlayer couldn't open H265 encoded files, but thankfully K-Lite Codecs did solve this.

It's obvious that BSPlayer has fallen behind. It really used to be in the front lines but it's more and more becoming obsolete.

Please update it to handle the latest codecs, multiple audio, and multiple subtitles