Hello, after a long time I realized what the issue was (still is).
In settings, subtitles are set to UTF-8. If the subtitle file is not in UTF-8 and contains special characters (Romanian in my case), the lines containing the characters don-t show on the screen. The lines that don't contain the special characters do show, though.
After converting the file to UTF-8 the subtitles work fine.
It would seem that problem is solved, but I wonder why it does that - mpvplayer plays the same files and with the same settings just fine, showing symbols instead of the special characters.