Well there I was typing a nice long response here when I clicked submit and oh bugger my session had expired!
Basically, I sorted it. Its related to my other post – https://generatepress.com/forums/topic/gp-child-theme-set-customizer-options-in-theme/
In my child theme I have a load of customizer options set via the functions.php with some using the generate_option_defaults filter.
It turns out when using the generate_option_defaults filter, any option I don’t specify is effectively blanked in the customizer, including the icon type.
So the solution is actually pretty simple, set the icon type via the generate_option_defaults filter.
Like so:
if ( !function_exists( 'gvhgroups_new_defaults' ) ) :
add_filter( 'generate_option_defaults','gvhgroups_new_defaults' );
function gvhgroups_new_defaults()
{
$new_defaults = array(
'icons' => 'svg'
);
return $new_defaults;
}
endif;
(Obviously my function had a load more array items in it!)
Thanks as ever for the help David!