Fixed width SRT files - Maintain white-space in SRT files
Sometimes text subtitles are formatted as closed captions - subtitles that are presented in fixed width. If a video container has CCs, you can extract the CC's as srt subtitles with CCExtractor_win_portable_v0.94, as an example.
Using MKVTOOLNIX 73.0.0, muxing fixed width srt files into an mkv causes most white-space to be stripped or chomped. It seems like all trailing white space is chomped, but not all leading white space is chomped. Ideally, unless the standard says otherwise, the subtitle text should not be changed at all - no chomping.
It is noted that many popular subtitle processing tools, such as Subtitle Edit, aegissub, and SubSync, do NOT preserve trailing white-space, and offer no options to alter that behavior.