The aerodynamics of driving with the tailgate up vs. tailgate off are totally different.
With the tailgate up, the air circulates in the bed and tends to create a bit of a vacuum in the front and a downdraft in the rear. That's why your coffee cups and leaves just blow around in the bed and why your bed mat tends to lift in the front even if it's a heavy rubber mat.
With the tailgate off, the air just flows straight through the bed, if anything, creating downforce on the mat. That's why it won't get sucked out unless it is very light.
The only bed mat I've ever had come close to sucking out of the truck was a light foam anti-fatigue floor mat that I was using as an inexpensive bed mat until I could afford something better. With the tailgate up and no tonneau cover, you had to have something heavy to hold it down or it would lift up in the front, get caught in the over-cab airflow, and head for the back of the bed.
