MKVToolNix does not show subtitles for some Media

Hello,

MKVToolNix does not show subtitles for some Media files. (They're usually MP4)

Screenshots: You can see the subtitles are available when playing with VLC or any other player. https://i.imgur.com/ldqsSh9.png

But.. as you can see from this screenshot MKVToolNix doesn't detect the subtitle. https://i.imgur.com/4XdnVPW.png

So can you fix this issue please? Thank you.