Visio Can't Undo Past Saves

If you make changes in a Visio document, and then save the document, suddenly you can't undo your changes anymore. I will save my normal long-winded derision by pointing out that this is an egregious, inexcusable, and unsurprising bug in Microsoft's software. Visio isn't the worse of Microsoft's software, but a lack of proper undo functionality means it sucks, and Microsoft is a failure.