Happened to me while I was adding every common word I could think of to my highlights. I was editing the highlights, then while editing them, pressed F9 and they disappeared. It also works with F8. Later I figured out that it actually replaces your highlights with what you have in osu! chat. So if you type something in chat, press F8, activate the highlights input box, then press F8 again, the text in osu! chat will be in your highlights.