From Editor Feedback Page:

when I try to use Clear Formatting in a "stuck" state like this, it will indeed remove the italics from whatever I had highlighted at that moment, but when I continue typing I'm still stuck in italics mode. Nothing will toggle off italics mode when this happens. Sure, the I button is no longer highlighted, but italics still appear when I type. Literally the only thing that works is to save the editing changes and then go back into the editor again.