Hi to All!
When “Automatically optimize when exporting” is selected for a PNG image it is converted to a JPG on export. (Hype v3.5.3  Pro). I’ve tested 6 different PNG images (no transparency) with the same outcome. This problem shows up in the Hype Preview as well.
I came across this issue because I was coding a background PNG image swap in an element and it of course requires a URL as part of the code. It took a while to track this one down.