Hey Ben,
The site title will only be an H1 if your home page is your posts page, as no other H1 exists on that page.
In this case, your front page is a static page, but it doesn’t have an H1.
You can use a filter to tell it to be an H1 like this:
add_filter( 'generate_site_title_output', function() {
return sprintf(
'<%1$s class="main-title" itemprop="headline">
<a href="%2$s" rel="home">
%3$s
</a>
</%1$s>',
( is_front_page() ) ? 'h1' : 'p',
esc_url( apply_filters( 'generate_site_title_href', home_url( '/' ) ) ),
get_bloginfo( 'name' )
);
} );
Adding PHP: https://docs.generatepress.com/article/adding-php/
Hope this helps ๐