Take a look in Abbott and Doenhoff, "Theory of Wing Sections". There are
plenty of airfoils with concave shape for the aft 25% of the section,
especially the symmetrical ones. It's the airfoil shape. If you fill it in,
you are changing the airfoil shape and no longer have the design airfoil.

Mike Reinath
LNC2 MKII @ 1050 hrs