Non-standard implementation error with custom configuration

I’m getting a strange “Non-standard implementation” error on my GA tag. I’ve got the plugin set up with the custom option, and am using the following function to insert the tag as suggested by the plugin:

function led_gtm4wp_insert () {
	if ( function_exists( 'gtm4wp_the_gtm_tag' ) ) {
		gtm4wp_the_gtm_tag();
	}
}
add_action('x_before_site_begin', 'led_gtm4wp_insert');

If you check out the site, you’ll see that the code gets inserted correctly, right after the first body tag- so I can’t work out why I’m getting the non-standard implementation error?

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