What happens if your student submits the answer AGAIN? Is it marked right again? If she continues to submit, can you get it to be marked wrong again?
If you CAN get it to be marked wrong, then could you edit the problem and change
ANS(fun_cmp($A*$C,var=>['h'],diagnostics=>1));and submit the problem until it fails to be marked correct. You should get some graphs and data at the bottom of the problem. Could you send me the data from the table below the graphs? (It should give the values of h used in the answer checking, plus the correct and student answers and the relative and absolute errors.)
The problem is written in a very old (pre-MathObjects) style, and it uses
fun_cmp even though the value is a number (presumably so that if the student enters answers that include h there is no error message). The problem should really be rewritten to use MathObject in Point context, but that is another matter.
fun_cmp is used, the problem compares the numbers as functions, and so produces several test points, and it seems that one or more of these is not matching for some reason (though I can't for the life of me think of why -- that is why I need the diagnostics output).
Finally, do you have anything in your course templates/macros directory?
Let's hope we can track this down.