I assume that you are describing the flexible fascia material as not holding a straight line or smooth arc, and sagging in places, rather than something caused by excessive sanding away of material?
I would think that a body shop could add a stiffening rib on the underside that would straighten out the waviness, if that is what you are describing. They would have to create a rib that is straight or curved the same as the way the backside of the fascia should be curved, and then glue it in place with professional autobody adhesive.
I’m sure that there are also other ways that they could think of doing it.