Restoring XtraBackup from MySQL 5.6 to MySQL 5.7
Xtrabackup is a physical backup tool, and is meant to backup and restore to the same MySQL (or MariaDB) version. It's not designed to work with migrations. It might work, it might not.

So if possible, if you want to restore the backup to a new instance, I think the best approach is to restore it to another MySQL 5.6 instance, and then upgrade that instance to 5.7 afterwards.