Steps to duplicate:
1. Create a Maintenance Plan with two steps.
2. Create a scheduled full backup, with an option to delete files older than 7 days.
3. In the same plan, scheduled differential backup, with an option to delete files older than 3 days.
When the plan runs, it will delete all files older then 3 days, regardless if they are full backups or differential backup.
Resolved in version 5.0.x
Each type of backup (Full, differential, transaction log) within the plan, has a separate sub-plan. These sub-plans control the schedule and option to remove old backup files.
Create a separate Maintenance Plan for each type of backup. One for Full backups and one for differential backups.