I wouldn't be worried about the curb as much as how low the screws are inside the pan, as the top of the curb isn't going to hold water like the pan will. The pan needs to be watertight to 2" above the curb by code where I am. If they have screws below that, the liner should be replaced and whoever put the board up should pay for it.

Just to confirm: the concrete under the liner isn't flat, right?