In "List of video containers"/"External subtitles"
I add a Matroska on list, srt subtitles and mkv movie works fine, when i remove it from the list it stop working.

I'm confused that list should contains a formats that NOT support external subtitles, so why when i add a Matroska format on it,
the external srt subtitles starts to work on .mkv movie ?
I do not use transcoding at all. Transcoding options are all set to off.