I'm glad I found this thread, my recently acquired Aura HD (running firmware 2.8.1) has the same issue the original poster has; I didn't notice it right away because I was usually fiddling with settings or browsing my library right off after opening the cover. Now that I've settled into just opening it just to read

(I know, such a novel concept with an ereader!) I would start reading only to have it go back to sleep; I did figure out that a tap on the screen was enough to keep it awake but still forget half the time.
I am relieved to know it's a common bug and not a flaw specific to my reader. And now I know that increasing the default sleep time is a simple fix for this (minor) frustration.