My instinct in looking at this is that it's not the sides of the bumper that need to go in, but the bottom of the bumper that needs to come down / back.
I bet when the bumper was installed that was all even, but when they tightened up the under tray and diffuser on the bottom it pulled the bumper upward from below and that's the natural strain direction in the material to push the corners outward. That's why it's so "springy". The bumper is rigid just like the other panels, it's not soft like a typical car bumper.