Mandatory reporters, not mandatory investigators. If they have no knowledge of the abuse they have nothing to report. If the girl says that her boyfriend is the father, the doctor isn't going to run extra tests just to prove her wrong. There is no medical advantage to the patient in such testing. Doctors are not cops.

Their primary directive legal or not is to uphold the oath they took to do no harm. If reporting would cause them to violate this ancient creed then it is for they to decide what they will do.

Sure, if they want to risk being stripped of their license, they can flout the law.

That law (in this case) is so difficult to enforce that it matters very little.