This is to document something I had to learn on my own and see if someone has a better solution.
I have a Movistar GT-I9300 Samsung S3 which I upgraded to carrier's bloated firmware 4.3 recently (could try the Samsung version but it has a different modem version).
After upgrading and installing some stuff I rooted and renamed bootsamsung.qmg (which is in fact Carrier's qmg).
After that, the screen did not turn off on its own or by pressing the Power Button. No way it could sleep / turn off / lock / etc.
I have not discovered this the easy way, but thats the story. Wakelock didn't help, sinice its not an App draining battery or preventing idle.
Doing so in 4.1.2 did not caused this issue.
Does anyone have a way of preventing the boot splash screen? Fortunately, renaming the PowerOn and PowerOff sound files mutes the noisy Boot and Shutdown.
PS: After I picked this up I noticed a small mention on a forum, but the main problem is that searching for this issue is not something that comes with relevant results.