Since this is the only site you're having problems with it might be related to the workpress plugin that is being used for posts/replys. According to the copyright at the bottom of this page that plugin is created by © Simple:Press —.
You might try looking at other forums where they are used, many listed here https://simple-press.com/showcase/.
Frustrating it is!
I had a look through my profile posting options, and changed from "bbCode - QuickTags" to "Rich text - TinyMCE" and that's put everything right.
It seems that (for me at least) the "bbCode - QuickTags" option doesn't work properly.