H3 and H4 font size options not showing (GP Premium)

Hi, for some reason, the h3 to h5 font size options are not showing in the customizer. I downloaded a completely blank starter child theme from the GeneratePress website. I've used GP Premium on several other sites and this is the first time this has happened.

To try and resolve this issue, I followed the "Manually Updating easy way" instructions here: https://docs.generatepress.com/article/updating-gp-premium/#manual-update. I've also reinstalled the theme but the problem persists.

How can I fix this issue? Many thanks!