I wondered this myself. Just to be clear, you all have the Auto Circulation box checked? Does it matter if you are using the Auto fan/system button above the Menu button? I never use that function.
I haven't had the opportunity to be behind a diesel truck yet, but I do go by a waste water treatment plant every week or so which will make you gag. Does it work on this type of stink or only particulates like soot or smoke?
And I believe we still have the automatic recirc function when you put the shifter in reverse, so you don't get your own exhaust fumes.