When a text field with a custom font is tied to a variable, interactively changing the text field causes the font to change to another font (when viewing a flow within the context of a pop-up viewer), sometimes with an erroneous “Missing font” error.
This bug has already lodged, and is infuriating plenty of people, but was automatically closed for no good reason:
Figma: When people take the time to lodge bugs for you, don’t reduce your opportunities to learn from them and provide a better product by closing them when there is no reason to close them.