The problem might be in the pose file. When you save a pose, not only do you have the option to save it con/sans morph channels, but you can select elements from a hierarchy type listing. It is allowed to add or not add the hair. I have not explored the results of each of the variables, but I suspect that if the hair were included in the pose file and a different hair model is present with the object, this may be what causes the problem. I do my poses with bald models and I think that the hair stays.