The way selecting a font-family (or the build in 'Styles') works for the text you have selected, is the same as, for example the 'Bold' or 'Italic' button. On the first activation it puts your selected text in the style/family you want, but if you press it again, it gets reset to what it was before your first activation.
For 'Bold' and 'Italic', this is expected, but in my opinion not for applying diffrent styles / font-families. (as a none experienced user might want to double check if the style he wants to apply is in fact active on his selected text). This also doesn't happen in editors like Pages or Word.
Is there any way i can disable the functionality of de-activating styles / font-families on a second press?