Typography - Font changes after navigating down from homepage

I'm using a code snippet to load Open Sans as a System Font, thinking that would speed up my pages somewhat:

add_filter( 'generate_typography_default_fonts','tu_add_system_fonts' );
function tu_add_system_fonts( $fonts ) {
    $fonts[] = 'Open Sans';
    return $fonts;
}

I used the Typography controls to set every single option to the Open Sans entry under System Fonts.

Everything works fine on the homepage, but once you navigate down one level the font appears to be some serif-looking font instead. Open Sans is still specified once I inspect, but not sure where this other font comes from.

Also, when I go back in Customizer the Open Sans is selected in the Google Fonts listing rather than System Fonts. Don't know if that's an issue or not.

So I guess I'm looking for a way that loads Opens Sans over the whole site in the fastest way possible. Also open to alternative methods.

Thanks,
Curt