IL Artboard not staying at final size after saving, closing, then opening

I have run into a strange problem where if I modify the size of the artboard for my file, using Document Setup, save, close, then reopen the file, it reverts back to its original artboard size.
But now I just found out that if I create a new document at a different size, then open a file, it changes the artboard to that new document size. Totally crazy.