Uninstalling facebook and facebook messenger fixes the problem
I've been having the issue even after updating to Lolipop so I figured maybe it's not TouchWiz, and maybe it's not Android, it might be an app. The issue on my S5 was that even during calls sometimes when I would take the phone away from my ear and the screen should light up so that I can close the call it would still just light up the softkeys. So I figured maybe it's an application that requires access to all privileges even the dialer. Also my phone was alaways connected to the internet and whenever I would have bad Mobile Network coverage I would notice the issue would manifest more frequently. So I thought directly to the Facebook apps who always get push notifications. I uninstalled both the Messenger and the Facebook app and for 3 days the "Soft keys on, screen not on" issue didn't manifest itself even once.
Hope this helps & best of luck to everybody,