If you highlight a section of text and apply styles to it like bold/italic/underline and place your cursor at the end of that text the Clear All Formatting button does not work. If you turn on those styles and then type some text and place your cursor at the end of that text it does work. The difference in the XAML is that in the first case the styles are only on the span of text itself while in the second the styles are on the span and the paragraph. I have attached a GIF of steps to reproduce and the XAML from the editor before clicking the Clear All Formatting button.