Backup Retention Not Working w/ B&R17 Free

The backup function has failed a few times due to the backup drive being full. In each situation I've found that the number of chains on the drive is in excess of the number of chains (2) set in the backup strategy. I'm forced to manually delete at least one chain to allow the backup to continue functioning. I've attached a screenshot of the settings for the backup strategy. Currently the backup contents shows 3 full backups, 6 increments for the first two full backups and 2 increments for the last full backup. Any suggestions on what to change to eliminate the need for the manual delete?

Please consider that keeping 2 chains means that the first chain will be removed after the third chain is completely finished.