Go to Settings > General > Keyboards and turn on Smart Punctuation. When I shut off Smart Punctuation, I got 2 dashes and when turned on, it showed the em dash in Messages.
If that does not solve the problem, you could try the âText Replacementâ option on that same settings page.
I was forced to use the reply box toolbar button for preformatted text since my keyboard doesnât have the grave accent but somehow it was also reformatted into an EM DASH after posting the reply. I donât know. I edited the initial post with a solution which could help. In my case it did, see --