Customer session being lost upon redirect from Payflow

Question

I am totally stumped! We have customers who incur multiple charges and never have their orders completed. There doesn't seem to be any rhyme or reason to the cause. There is no consistent pattern. In fact, we just had a customer who was logged out (and charged). The customer immediately logged back in, and the order was successful (charged again). We have determined that the session of affected customers seems to be newly established upon redirect from Payflow. We cannot figure out the cause. I cannot reproduce this in development, no matter how many times I've tried. We are using the silent post method. Our sites run HTTPS-only, so there's no mixed protocol. Also ensured there is no mixed domain between www and non-www. I am at a complete loss. Any and all assistance is appreciated. Thank you!