SSR: New Monthly Backup Wiped Out Previous Backups - Why?

Second month in a row this has happened on wife's Win 10 PC - running SSR 2013 R2:  SSR runs a scheduled full backup with a new restore point on the first of the month; however, in so doing all of the previous backups/restore points are being deleted.  As a result, SSR now shows only yesterday's 1 Sept new backup/restore point; all of August's backups have been deleted. Backup settings call for 2 recovery point sets to be retained. The backup drive is a 3TB external USB drive. The new backup restore point consumed about 850GB, and the pie chart on the Home page showed about 2TB were free before the new backup ran and still shows about 2TB free after the backup. Any ideas why the previous backups are being deleted? Thanks.