Thanks for pointing this out, I have logged a bug for it. Unlike the Contact Form Solution Pack CSS Form Builder does not set the entered answer to lowercase before making the comparison so entering the capital letter will cause it to fail validation.
The work around is to update the security question validation on the page and make it look like this:
$WAFV_Errors .= WAValidateLE((isset($_POST["Registration_group_field"])?strtolower($_POST["Registration_group_field"]):"") . "",((isset($_SESSION["random_answer"]))?strtolower($_SESSION["random_answer"]):"") . "",true,13);
I just added in the strtolower() around the posted element to make it lowercase and match the session variable.