Google IPS retention issues and you'll find plenty of them. It can happen after seconds - not 8 hours. My iPad 2, after a year of owning it would show retention if I left anything on the display for 30-60 seconds. Took it into Apple, it was instantly recognized by the tech and replaced.
Lots of IPS displays have had this problem, just not all IPS displays.
That's clearly a defective display.
IPS as a technology does NOT burn in unless left on for several hours.