Problem with purging an old backup

Hi,
in the advanced settings I can set, that I want to hold only one backup and purge the older ones before or after starting the new backup.
This is important, because perhaps the volulme has got 4GB free and this is too little.
Therefore macrium has to delete the older backup prior.

But if I start the backup, I see the older backup in my windows-explorer and macrium hasn't deleted the older backup.

Why? Is that a bug? Or what can I do to set this function so, that it works?

Thanks,
Thomas