Please feel free to share. “Amazing Grace”
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.