For some reason, LuaLaTeX is outputting an en-dash for both --
and ---
for me.
MWE:
test.tex:
\documentclass{article}
\begin{document}
blah-blah
blah--blah
blah---blah
\end{document}
lualatex test
produces:
pdflatex test
produces:
---
into Google would not be helpful here ;-) (It is bad enough if you search for more recognisable code with a few more letters.) I only found the duplicate so quickly because I knew I had seen it before and could search in my browsing history for "em" and "Lua". Would you be OK with closing this as a duplicate?