diff --git a/wp-admin/edit-form-blocks.php b/wp-admin/edit-form-blocks.php
index 9e26086849..7b80b304f1 100644
--- a/wp-admin/edit-form-blocks.php
+++ b/wp-admin/edit-form-blocks.php
@@ -358,10 +358,6 @@ do_action( 'enqueue_block_editor_assets' );
 require_once( ABSPATH . 'wp-admin/includes/meta-boxes.php' );
 register_and_do_post_meta_boxes( $post );
 
-// Some meta boxes hook into the 'edit_form_advanced' filter.
-/** This action is documented in wp-admin/edit-form-advanced.php */
-do_action( 'edit_form_advanced', $post );
-
 require_once( ABSPATH . 'wp-admin/admin-header.php' );
 ?>
 
diff --git a/wp-admin/includes/post.php b/wp-admin/includes/post.php
index ced29d4a72..aba113c7a5 100644
--- a/wp-admin/includes/post.php
+++ b/wp-admin/includes/post.php
@@ -2307,6 +2307,43 @@ function the_block_editor_meta_box_post_form_hidden_fields( $post ) {
 	$current_user = wp_get_current_user();
 	$user_id      = $current_user->ID;
 	wp_nonce_field( $nonce_action );
+
+	/*
+	 * Some meta boxes hook into these actions to add hidden input fields in the classic post form. For backwards
+	 * compatibility, we can capture the output from these actions, and extract the hidden input fields.
+	 */
+	$actions = array(
+		'edit_form_after_title',
+		'edit_form_advanced',
+	);
+
+	foreach ( $actions as $action ) {
+		ob_start();
+		do_action_deprecated(
+			$action,
+			array( $post ),
+			'5.0.0',
+			'block_editor_meta_box_hidden_fields',
+			__( 'This action is still supported in the classic editor, but is deprecated in the block editor.' )
+		);
+		$classic_output = ob_get_clean();
+
+		if ( ! $classic_output ) {
+			continue;
+		}
+
+		$classic_elements = wp_html_split( $classic_output );
+		$hidden_inputs    = '';
+		foreach( $classic_elements as $element ) {
+			if ( 0 !== strpos( $element, '<input ') ) {
+				continue;
+			}
+
+			if ( preg_match( '/\stype=[\'"]hidden[\'"]\s/', $element ) ) {
+				echo $element;
+			}
+		}
+	}
 	?>
 	<input type="hidden" id="user-id" name="user_ID" value="<?php echo (int) $user_id; ?>" />
 	<input type="hidden" id="hiddenaction" name="action" value="<?php echo esc_attr( $form_action ); ?>" />
@@ -2324,4 +2361,16 @@ function the_block_editor_meta_box_post_form_hidden_fields( $post ) {
 	wp_nonce_field( 'closedpostboxes', 'closedpostboxesnonce', false );
 	// Permalink title nonce.
 	wp_nonce_field( 'samplepermalink', 'samplepermalinknonce', false );
+
+	/**
+	 * Add hidden input fields to the meta box save form.
+	 *
+	 * Hook into this action to print `<input type="hidden" ... />` fields, which will be POSTed back to
+	 * the server when meta boxes are saved.
+	 *
+	 * @since 5.0.0
+	 *
+	 * @params WP_Post $post The post that is being edited.
+	 */
+	do_action( 'block_editor_meta_box_hidden_fields', $post );
 }
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 07041fc06d..4370a3dabe 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.1-alpha-44240';
+$wp_version = '5.1-alpha-44241';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.