I was just pulling a guess out of... well, anyway.
Dumb stuff like that has happened to me before.
"Oh, I will change the font- now isn't that better? Yeah, now I don't need my glasses as much.
Hey, where the heck did that other setting go? Why do I have to scroll where I never had to before?"
Now add on a busy schedule and a pile of forgetfulness.
I have not had a phone with a short time out limit since my ZTE 432 (Blackberry knockoff), and the limit was like 30 seconds!
I just use 5 minutes now, it seems fine on battery and no unexpected screen offs.
I use Greenify so that I can turn the screen off with a long press of the Assistant button, and if I am out and about I use Proximity Service to shut the screen off when I put the device inside its case.
Greenify closes all (selected) apps and then locks the screen, and Proximity Service only turns off the screen, and has a wakelock- so that when I pull the phone back out it comes on (because of Wave Up) and is right where it was when I put it inside the case.
So unless I leave the device unattended, it is rare that the timer actually gets to do anything anyway.