diff --git a/src/wp-includes/class-wp-duotone.php b/src/wp-includes/class-wp-duotone.php
index d0b978eacef21..b75b01619fbee 100644
--- a/src/wp-includes/class-wp-duotone.php
+++ b/src/wp-includes/class-wp-duotone.php
@@ -1191,8 +1191,7 @@ public static function restore_image_outer_container( $block_content ) {
 		$tags->set_bookmark( 'wrapper-div' );
 		$tags->next_tag();
 
-		$inner_classnames = explode( ' ', $tags->get_attribute( 'class' ) );
-		foreach ( $inner_classnames as $classname ) {
+		foreach ( $tags->class_list() as $classname ) {
 			if ( str_starts_with( $classname, 'wp-duotone' ) ) {
 				$tags->remove_class( $classname );
 				$tags->seek( 'wrapper-div' );
diff --git a/tests/phpunit/tests/block-supports/duotone.php b/tests/phpunit/tests/block-supports/duotone.php
index 808903a072452..fef2e74929286 100644
--- a/tests/phpunit/tests/block-supports/duotone.php
+++ b/tests/phpunit/tests/block-supports/duotone.php
@@ -61,6 +61,18 @@ public function test_render_duotone_support_custom() {
 		$this->assertMatchesRegularExpression( $expected, WP_Duotone::render_duotone_support( $block_content, $block, $wp_block ) );
 	}
 
+	/**
+	 * @covers ::restore_image_outer_container
+	 */
+	public function test_restore_image_outer_container_moves_duotone_class_to_wrapper_in_classic_theme() {
+		switch_theme( 'default' );
+
+		$block_content = '<div class="wp-block-image"><figure class="alignright wp-duotone-blue-orange size-full"><img src="/my-image.jpg"></figure></div>';
+		$expected      = '<div class="wp-block-image wp-duotone-blue-orange"><figure class="alignright size-full"><img src="/my-image.jpg"></figure></div>';
+
+		$this->assertEqualHTML( $expected, WP_Duotone::restore_image_outer_container( $block_content ) );
+	}
+
 	/**
 	 * Tests whether the slug is extracted from the attribute.
 	 *