Did you happen to copy and paste some of this text in from somewhere else? Many customers write in Google Docs or Microsoft Word first then move their content into beehiiv.
When you copy/paste text, it can keep those old styles from where you originally copied it from which can include styles for setting the font.
You can remove the inline styles by highlighting the affected paragraph and then resetting all of the styles for it (image below).
The slider icon on the right of the menu is highlighted pink, this is an indicator that the highlight text has some styles applied to it. Once you reset it, it will use the font you’ve chosen from your Design Lab..