I am experiencing the same issue with twodifferent kinds of question:
1) I have set up a multiple numerical input question so that the people who decide to answer must distribute 100 points (%) into a few categories (equals_num_value = 100). This question is set up as not mandatory. However when one decides to skip it and go on, a warning saying that the question must be properly answered appears. In this way it is not possible to go on without answering this question, which is what I don't want. I would like people to be free to answer or not this question. Those who decide to answer must answer properly, that is they must distribute 100 points into given categories;
2) the same thing happens in a ranking question. The question is not mandatory: so the people who decide to answer must rank all the listed categories (7), while those who decide to skip it must have the chance to go on without answering.