One way sync deleting files on backup destination, it shouldn't

I am using HBS to do a one way sync from my main NAS to my old NAS and what is happening when I delete a file on the main, it is being deleted on the old.

The setting "Remove additional files in destination folder" is turned off so this behaviour is unexpected.

Can someway tell me how to stop the backup deleting files simply because I remove them from the main, it totally defeats the purpose of the sync.