Source

File: wp-includes/rest-api/fields/class-wp-rest-meta-fields.php

protected function update_meta_value( $object_id, $meta_key, $name, $value ) {
		$meta_type = $this->get_meta_type();

		if ( ! current_user_can( "edit_{$meta_type}_meta", $object_id, $meta_key ) ) {
			return new WP_Error(
				'rest_cannot_update',
				/* translators: %s: Custom field key. */
				sprintf( __( 'Sorry, you are not allowed to edit the %s custom field.' ), $name ),
				array(
					'key'    => $name,
					'status' => rest_authorization_required_code(),
				)
			);
		}

		// Do the exact same check for a duplicate value as in update_metadata() to avoid update_metadata() returning false.
		$old_value = get_metadata( $meta_type, $object_id, $meta_key );
		$subtype   = get_object_subtype( $meta_type, $object_id );

		if ( 1 === count( $old_value ) && $this->is_meta_value_same_as_stored_value( $meta_key, $subtype, $old_value[0], $value ) ) {
			return true;
		}

		if ( ! update_metadata( $meta_type, $object_id, wp_slash( $meta_key ), wp_slash_strings_only( $value ) ) ) {
			return new WP_Error(
				'rest_meta_database_error',
				/* translators: %s: Custom field key. */
				sprintf( __( 'Could not update the meta value of %s in database.' ), $meta_key ),
				array(
					'key'    => $name,
					'status' => WP_Http::INTERNAL_SERVER_ERROR,
				)
			);
		}

		return true;
	}