Interestingly, in my client both show up as separate dashes. I suspect it’s simply a ligature in the font used in your frontend.
E: Hm, no that is not it, copy-pasting the text from the web frontend copies the em- an en-dashes, so it’s actually replacing the characters during rendering.
With that you made an en-dash, not an em-dash.
Let’s see if it does anything if you use three a — b vs two a – b ?
Edit: Yep looks like that’s it.
Interestingly, in my client both show up as separate dashes. I suspect it’s simply a ligature in the font used in your frontend.
E: Hm, no that is not it, copy-pasting the text from the web frontend copies the em- an en-dashes, so it’s actually replacing the characters during rendering.