I don't understand: æ is not a ligature so it's not an example of a ligature. There are English ligatures to use.

Also, most fonts have many characters beyond ASCII, including æ. If your font lacked it then you would see an empty box, not the two letters ae. Applying a font style would not change the rendering of æ into ASCII letters; I don't think it changes the rendering of English ligatures, which are separate code points in Unicode.