src/admin/class-papi-admin-meta-handler.php
public function save_revision( $revision_id ) {
		// Check if our nonce is vailed.
		if ( ! wp_verify_nonce( papi_get_sanitized_post( 'papi_meta_nonce' ), 'papi_save_data' ) ) {
			return;
		}

		// Fetch parent id from revision.
		if ( ! $parent_id = wp_is_post_revision( $revision_id ) ) {
			return;
		}

		// Bail if a entry type don't exists on parent post.
		if ( papi_is_empty( papi_get_entry_type_by_meta_id( $parent_id, 'post' ) ) ) {
			return;
		}

		$meta = get_post_meta( $parent_id );

		foreach ( $meta as $key => $value ) {
			if ( $key[0] === '_' && $key !== papi_get_page_type_key() ) {
				continue;
			}

			papi_data_update( $revision_id, $key, array_shift( $value ) );
		}
	}
public function save_revision( $revision_id ) {
		// Check if our nonce is vailed.
		if ( ! wp_verify_nonce( papi_get_sanitized_post( 'papi_meta_nonce' ), 'papi_save_data' ) ) {
			return;
		}

		// Fetch parent id from revision.
		if ( ! $parent_id = wp_is_post_revision( $revision_id ) ) {
			return;
		}

		// Bail if a entry type don't exists on parent post.
		if ( papi_is_empty( papi_get_entry_type_by_meta_id( $parent_id, 'post' ) ) ) {
			return;
		}

		$meta = get_post_meta( $parent_id );

		foreach ( $meta as $key => $value ) {
			if ( $key[0] === '_' && $key !== papi_get_page_type_key() ) {
				continue;
			}

			papi_data_update( $revision_id, $key, array_shift( $value ) );
		}
	}
Save custom fields for revision post.

@param int $post_id