As general advice, if a measure is behaving in a irregular way, the best way to debug is to first find a concise and clear example for which you can easily determine what the correct value from the measure should be, such as selecting a specific order or account or product combined with a month, whatever is most clear. Then, strip the measure of any extras, set analysis/ifs/formatting and just pure Sum(Field). Assuming that simple measure is working correctly, i.e. there's no issue in the actual fact data in the data model, you can start adding stuff back in like the set analysis one step at the time, and see at what point you start getting issues. That should help you locate your problem and give clarity how to solve it.
