@MarkTurkenburg - I don't know if this will work or not, but whenever this happens to me, I click the remove sort button in the bottom right corner of your screenshot and then try to re-apply the sort after that. Not sure if that will solve your issues or not, but it usually works for me.
Thanks @jasonblinn - I think your response would work in most situations
FYI for everyone: The reason for the sorting issue seems to be because the Line Item was a subsidiary view. I created a replica line item in another module with the same dimensionality & referenced the original data and was able to sort