diff --git a/lib/class-wp-theme-json-resolver-gutenberg.php b/lib/class-wp-theme-json-resolver-gutenberg.php
index 9b39213fd14226..76999b6ac34366 100644
--- a/lib/class-wp-theme-json-resolver-gutenberg.php
+++ b/lib/class-wp-theme-json-resolver-gutenberg.php
@@ -245,9 +245,6 @@ public static function get_theme_data( $deprecated = array(), $options = array()
 			} else {
 				$theme_json_data = array();
 			}
-			// BEGIN OF EXPERIMENTAL CODE. Not to backport to core.
-			$theme_json_data = gutenberg_add_registered_fonts_to_theme_json( $theme_json_data );
-			// END OF EXPERIMENTAL CODE.
 
 			/**
 			 * Filters the data provided by the theme for global styles and settings.
@@ -266,10 +263,7 @@ public static function get_theme_data( $deprecated = array(), $options = array()
 				if ( '' !== $parent_theme_json_file ) {
 					$parent_theme_json_data = static::read_json_file( $parent_theme_json_file );
 					$parent_theme_json_data = static::translate( $parent_theme_json_data, $wp_theme->parent()->get( 'TextDomain' ) );
-					// BEGIN OF EXPERIMENTAL CODE. Not to backport to core.
-					$parent_theme_json_data = gutenberg_add_registered_fonts_to_theme_json( $parent_theme_json_data );
-					// END OF EXPERIMENTAL CODE.
-					$parent_theme = new WP_Theme_JSON_Gutenberg( $parent_theme_json_data );
+					$parent_theme           = new WP_Theme_JSON_Gutenberg( $parent_theme_json_data );
 
 					/*
 					 * Merge the child theme.json into the parent theme.json.
@@ -279,6 +273,11 @@ public static function get_theme_data( $deprecated = array(), $options = array()
 					static::$theme = $parent_theme;
 				}
 			}
+
+			// BEGIN OF EXPERIMENTAL CODE. Not to backport to core.
+			static::$theme = gutenberg_add_registered_fonts_to_theme_json( static::$theme );
+			// END OF EXPERIMENTAL CODE.
+
 		}
 
 		if ( ! $options['with_supports'] ) {
diff --git a/lib/experimental/fonts-api/register-fonts-from-theme-json.php b/lib/experimental/fonts-api/register-fonts-from-theme-json.php
index cf943111963fdd..1038af2956a69b 100644
--- a/lib/experimental/fonts-api/register-fonts-from-theme-json.php
+++ b/lib/experimental/fonts-api/register-fonts-from-theme-json.php
@@ -121,8 +121,11 @@ function gutenberg_register_fonts_from_theme_json() {
 	 */
 	function gutenberg_add_registered_fonts_to_theme_json( $data ) {
 		$font_families_registered = wp_fonts()->get_registered_font_families();
-		$font_families_from_theme = ! empty( $data['settings']['typography']['fontFamilies'] )
-			? $data['settings']['typography']['fontFamilies']
+
+		$raw_data = $data->get_raw_data();
+
+		$font_families_from_theme = ! empty( $raw_data['settings']['typography']['fontFamilies']['theme'] )
+			? $raw_data['settings']['typography']['fontFamilies']['theme']
 			: array();
 
 		/**
@@ -157,21 +160,24 @@ function gutenberg_add_registered_fonts_to_theme_json( $data ) {
 
 		// Make sure the path to settings.typography.fontFamilies.theme exists
 		// before adding missing fonts.
-		if ( empty( $data['settings'] ) ) {
-			$data['settings'] = array();
+		if ( empty( $raw_data['settings'] ) ) {
+			$raw_data['settings'] = array();
+		}
+		if ( empty( $raw_data['settings']['typography'] ) ) {
+			$raw_data['settings']['typography'] = array();
 		}
-		if ( empty( $data['settings']['typography'] ) ) {
-			$data['settings']['typography'] = array();
+		if ( empty( $raw_data['settings']['typography']['fontFamilies'] ) ) {
+			$raw_data['settings']['typography']['fontFamilies'] = array();
 		}
-		if ( empty( $data['settings']['typography']['fontFamilies'] ) ) {
-			$data['settings']['typography']['fontFamilies'] = array();
+		if ( empty( $raw_data['settings']['typography']['fontFamilies'] ) ) {
+			$raw_data['settings']['typography']['fontFamilies']['theme'] = array();
 		}
 
 		foreach ( $to_add as $font_family_handle ) {
-			$data['settings']['typography']['fontFamilies'][] = wp_fonts()->to_theme_json( $font_family_handle );
+			$raw_data['settings']['typography']['fontFamilies']['theme'][] = wp_fonts()->to_theme_json( $font_family_handle );
 		}
 
-		return $data;
+		return new WP_Theme_JSON_Gutenberg( $raw_data );
 	}
 }