Index: src/wp-includes/meta.php
===================================================================
--- src/wp-includes/meta.php	(revision 38040)
+++ src/wp-includes/meta.php	(working copy)
@@ -1032,6 +1032,7 @@
 
 	// There used to be individual args for sanitize and auth callbacks
 	$has_old_sanitize_cb = false;
+	$has_old_auth_cb = false;
 
 	if ( is_callable( $args ) ) {
 		$args = array(
@@ -1045,6 +1046,7 @@
 
 	if ( is_callable( $deprecated ) ) {
 		$args['auth_callback'] = $deprecated;
+		$has_old_auth_cb = true;
 	}
 
 	$args = wp_parse_args( $args, $defaults );
@@ -1062,7 +1064,7 @@
 	$args = apply_filters( 'register_meta_args', $args, $defaults, $object_type, $meta_key );
 
 	// Object subtype is required if using the args style of registration
-	if ( ! $has_old_sanitize_cb && empty( $args['object_subtype'] ) ) {
+	if ( ! $has_old_sanitize_cb && ! $has_old_auth_cb && empty( $args['object_subtype'] ) ) {
 		return new WP_Error( 'register_meta_failed', __( 'Meta must be registered against an object subtype.' ) );
 	}
 
@@ -1081,11 +1083,16 @@
 		$object_subtype = $args['object_subtype'];
 	}
 
-	// Back-compat: old sanitize and auth callbacks applied to all of an object type
-	if ( $has_old_sanitize_cb ) {
+	// Back-compat: old sanitize and auth callbacks are applied to all of an object type.
+	if ( $has_old_sanitize_cb && is_callable( $args['sanitize_callback'] ) ) {
 		add_filter( "sanitize_{$object_type}_meta_{$meta_key}", $args['sanitize_callback'], 10, 4 );
+	}
+
+	if ( $has_old_auth_cb && is_callable( $args['auth_callback'] ) ) {
 		add_filter( "auth_{$object_type}_meta_{$meta_key}", $args['auth_callback'], 10, 6 );
-	} else {
+	}
+
+	if ( ! $has_old_auth_cb && ! $has_old_sanitize_cb) {
 		if ( is_callable( $args['sanitize_callback'] ) ) {
 			add_filter( "sanitize_{$object_type}_{$object_subtype}_meta_{$meta_key}", $args['sanitize_callback'], 10, 4 );
 		}
Index: tests/phpunit/tests/meta/registerMeta.php
===================================================================
--- tests/phpunit/tests/meta/registerMeta.php	(revision 38040)
+++ tests/phpunit/tests/meta/registerMeta.php	(working copy)
@@ -32,10 +32,42 @@
 		return $meta_key . ' new sanitized';
 	}
 
+	public function _old_auth_meta_cb( $allowed, $meta_key, $post_id, $user_id, $cap, $caps ) {
+		return $allowed;
+	}
+
 	public function _new_auth_meta_cb( $allowed, $meta_key, $post_id, $user_id, $cap, $caps ) {
 		return $allowed;
 	}
 
+	public function test_register_meta_back_compat_with_auth_callback_and_no_sanitize_callback_has_old_style_auth_filter() {
+		register_meta( 'post', 'flight_number', null, array( $this, '_old_auth_meta_cb' ) );
+		$has_filter = has_filter( 'auth_post_meta_flight_number', array( $this, '_old_auth_meta_cb' ) );
+		remove_filter( 'auth_post_meta_flight_number', array( $this, '_old_auth_meta_cb' ) );
+
+		// The filter should have been added with a priority of 10.
+		$this->assertEquals( 10, $has_filter );
+	}
+
+	public function test_register_meta_back_compat_with_sanitize_callback_and_no_auth_callback_has_old_style_sanitize_filter() {
+		register_meta( 'post', 'flight_number', array( $this, '_old_sanitize_meta_cb' ) );
+		$has_filter = has_filter( 'sanitize_post_meta_flight_number', array( $this, '_old_sanitize_meta_cb' ) );
+		remove_filter( 'sanitize_post_meta_flight_number', array( $this, '_old_sanitize_meta_cb' ) );
+
+		$this->assertEquals( 10, $has_filter );
+	}
+
+	public function test_register_meta_back_compat_with_auth_and_sanitize_callback_has_old_style_filters() {
+		register_meta( 'post', 'flight_number', array( $this, '_old_sanitize_meta_cb' ), array( $this, '_old_auth_meta_cb' ) );
+		$has_filters = array();
+		$has_filters['auth'] = has_filter( 'auth_post_meta_flight_number', array( $this, '_old_auth_meta_cb' ) );
+		$has_filters['sanitize'] = has_filter( 'sanitize_post_meta_flight_number', array( $this, '_old_sanitize_meta_cb' ) );
+		remove_filter( 'auth_post_meta_flight_number', array( $this, '_old_auth_meta_cb' ) );
+		remove_filter( 'sanitize_post_meta_flight_number', array( $this, '_old_sanitize_meta_cb' ) );
+
+		$this->assertEquals( array( 'auth' => 10, 'sanitize' => 10 ), $has_filters );
+	}
+
 	public function test_register_meta_with_valid_object_type_and_object_subtype_returns_true() {
 		$result = register_meta( 'post', 'flight_number', array( 'object_subtype' => 'post' ) );
 		unregister_meta_key( 'post', 'post', 'flight_number' );