Set default blocks in ACF Flexible Field

add_filter('acf/load_value/name=flexible_content', function ($value, $post_id, $field) {

	// Make sure it's not an existing post item
	if ($value !== NULL) : return $value;
	endif;

	// Set default layout
	$value = [
		['acf_fc_layout' => 'LAYOUT_SLUG']
	];

	return $value;
}, 10, 3);