Internal heuristics to remove non-dialogue lines should include subtitles lines enclosed entirely in brackets.

[gamagoat]

Example of such a line:

[Johnny humming.]

Shows, at least those on Netflix, use the above standard for non-dialogue subtitles. I currently manually remove them via -R "^(\[(.*?)\])$", but believe this could be part of the standard "internal heuristics" to remove non-dialogue subtitles.