Hi Patrick,
If you do a online restore in the Mendix cloud as described here(3.2.1, you won't lose any data: https://docs.mendix.com/developerportal/howto/how-to-restore-a-backup
Im guessing you do this restore to achieve something like this?: https://docs.mendix.com/howtogeneral/support/database-maintenance-size-reduction#2-2-option-2-creating-and-restoring-a-backup-of-the-database