First of all, what version of SmarterMail are you running? I've tested this on SM 15.x and it does look like something weird is going on.
Setting the Deletion Bounce Threshold creates a dynamic sending address. In your case, "firstname.lastname@example.org". After X number of bounces, the subscriber should be removed from the mailing list and the bounce returned to the List From Address. If the Deletion Bounce Threshold is disabled, then any bounces should return to the List From Address.
I first set the Deletion Bounce Threshold on my test list and saw the same results as you: messages bounce back to the dynamic address, but nothing came to the List From. I disabled the Deletion Bounce Threshold BUT I see the same results.
Just to be safe, I created an entirely new mailing list after disabling the DBT, but the same issues occur. So...I will have to work through this with the developers to see what's going on.