We have two options to replace -- by EN (or EM) DASH, one in Tools > Autocorrect: Replace (-- = -), the other at Options ("[ ] Replace dashes"). This clearly leads to confusion.
Auto-correction/Replace is defined in extras/autocorr/lang/<id>/Documentlist.xml and turns two dashes into one EN DASH U+2013 for more or less all languages (grep -R – *), many but not all replace three dashes by EM DASH U+2014 (grep -R — *). NL and NL_BE replace a single dash by EN DASH.
The "option" replacement is done via SvxAutoCorrect::DoAutoCorrect() at IsAutoCorrFlag(ACFlags::ChgToEnEmDash) and SvxAutoCorrect::FnChgToEnEmDash(), which is a rather complex function.
Now I'd vote for removing the option from the replacement table as this is in my skills but I'm afraid of the triple dash thing / EM dash. At least for me it is not converted after removing the replacement entry. Removing the code in SvxAutoCorrect might be doable but could have side-effects.
Seeking for opinions here (x-post to dev and l10n).
BZ Ticket is https://bugs.documentfoundation.org/show_bug.cgi?id=142938