If I have bolded or italicized Markdown text, and I place my cursor inside that text (with no text selected), I expect Ctrl-B or Ctrl-I to unbold or unitalicize the text.
Currently it adds more asterisks, as if to let you start typing bold or italic text. (Of course, that does not even work, it just breaks the formatting.)
This is how most other Markdown editors work, in my experience. It makes unformatting text much easier. You no longer have to carefully select the text to unformat, you just have to put the cursor inside it.