If I group by due-date today (Sept.17), the last-week group is titled "Last week (Sep 10 - Sep 16)".
I think MLO is considering "last week" to be the last 7 days, not necessarily the last calendar week.
What might be a minor bug is that Sep 16 due dates are actually included in the "Yesterday" group, not in the "Last week (Sep 10 - Sep 16)" group.