Unable to resume encrypted backups

I have been unable to resume a partial backup. There are two use cases:
1) While running a backup I press the "Resume later" button.
2) Backup fails due to "connection timed out".

In both cases I can see the files that have been added to the backup directory by the halted backup process. The next time I use DD and press "Backup now" I expect the backup to continue from where it left off (with the possible exception of rolling back any files that were only partly backed up). However, what happens is that DD deletes all the files from the previous session and starts from the beginning again.

(BTW these are encrypted backups. I have not tried it without encryption.)

Reproduce the problem:
Get the starting count of files in the backup directory.
Run backup (In my case, always with encryption, and always over a LAN.)
Stop the backup part way through (either by pressing "Resume later" or breaking the network connection)
Get the count of files in the backup directory (should be more than the starting count)
Run backup again (press "Backup now")
Get the count of files in the backup directory once the backup has started uploading (For me the file count has been taken back to the starting count)