Because the operation you are doing, somehow, does not allow Aurelius to know the old state of the object. For example, that can happen if you use Manager.Update(SomeObject). Is that the case?
I'm sorry, but that looks like the same problem you reported with the Delete. That simply works, I retested it here.
Your code, somehow, is "removing" the object from the manager, or it's not including it at all. Thus the dataset is calling Manager.Update and including the object in the manager directly, that's the reason why you don't have ChangedColumnNames and other properties that indicate previous state.
But that happens because when you call Post, your object is not in the manager. I don't know why, you have to review your code or try to send us a project that reproduces the problem.