Confirmed!
Sorry , but your post did mention {-nsub or key 'S'}, which caused confusion.
Meanwhile created a playlist with 2CD's with "samename".srt-subtitles and tested starting up playlist with commandline ending at -nsub.
It seems -nsub in commandline indeed only works once: either for a single videofile started up by commandline; or for the
first videofile in a playlist-file started up by commandline.
So if you don't want subtitles for files in a playlist, you must have checked/selected "Subtitles disabled" under
(rightclick >) Options > Preferences > Subtitles
before using commandline with playlist-file.