Welcome to the forum @Yves_Smolders
To my understanding, it’s not. It backs up while finding files. You wouldn’t want to wait before uploading…
Ransomware Detection with SyncBack describes a % of files have changed based approach, along with another approach based on change in a file of your choice that should NOT change, similarly to this topic which would let you write this yourself using run-script-before to do what you like, including don’t back up.
I worry about that plan, because I don’t think the ransomware is guaranteed to clobber chosen file first…
Ransomware that gets to either Duplicati program or its database is likely to halt the backup at that point. Admittedly there is a risk of a messed up backup (which would be unfortunate) if a hard stop messes up.
According to the comparison chart, the feature is present even in SyncBack Free which lacks versioning. That’s probably where it’s most useful. True versioned backups could just restore from an earlier version, provided ransomware didn’t get to the backup (which seems a bigger risk if backup goes to ordinary files.
There are some other posts where people are busy figuring out the best way to protect their backup files.