S3 backup not rotated properly?

I’ve set my MiaB box to do backups to S3. So far everything has worked fine, but yesterday I noticed that it didn’t delete the previous backup files, even if MiaB did a full backup.

10/17/16 03:00:07 EEST 17 hours, 26 minutes ago increment 1.1 MB approx. 30 days
10/16/16 03:00:07 EEST 1 day, 17 hours ago full 1.5 GB approx. 30 days
10/15/16 03:00:05 EEST 2 days, 17 hours ago increment 1.3 MB 6 hours, 33 minutes

Why didn’t it delete the backups and cleared the backup log? What’s the logic for when the backup is deleted?