This issue just started happening for me. I've unistalled and reinstalled Illustrator, created and test saved multiple times to attempt to duplicate the issue. It seems to only be an issue with PNG's, placing Jpgs doesn't have the same result. When saving as a PDF the issue doesn't not occure. It only occurs when savign as an .ai file.
I have other options to create similar files but I liked using illustrator and not knowing what changed to cause this issue is annoying.