Write some text into the Rich Text editor and then select a space between two words. Choose Insert -> Symbol and then use mdash. It renders correctly. Save the page and look at what you get and it changes to a series of hyphens. Edit again and look at Wiki Code. Same thing happens with a space between the words and the mdash but it changes to three hyphens. The correct use of the mdash is to use it without spaces.

I looked in the database and there is an issue already reported for this and labeled fixed. It's CONF-501 and it's 6 years old. Perhaps this is a regressed bug?