So you are jumping into the reply by hitting the quote button in a thread?

If not, let me know the entry point or exactly what you are doing.

It looks like you aren't in bbcode mode and somehow the javascript code that manages the editor is having an issue.

Try to set it to bbcode mode (click that button or set as your default) and see if it still does the same thing. It's strange that the opening tag is missing, but from your image, you are in the visual editor, yet you have the closing tag as a bbcode tag. It's like it's half working right, which makes no sense. Especially if this happens to you all the time.