Yes, I have the same problem on my Inkpad 3, but I also haven't been able the find its cause. PB support and a PB representative at e-reader-forum.de both suggested that it may be a hardware issue, and I should send the device in. (I did not until now, but I probably will at some point before the end of warranty.)
My first suspect was also Koreader, but the devs said it's nonsense. (
https://www.mobileread.com/forums/sh...d.php?t=316768) Just out of curiosity: have you reinstalled Koreader after the reset & reformat? Or did anything change with a fresh, Koreader-less install?