Finally decided to take my SM-G900F to Samsung here in Romania where I got it, told the guys I tried everything: factory reset, wipe cache, whatever and they said to leave it there for some thorough tests. Next day they call me to get my phone back, the screen was replaced and I was happy I didn't get the issue for like a whole day, yesterday (please keep in mind that Samsung made no factory reset on my phone, when I got it back it still had all my apps installed).
However I just got the "screen off softkeys on" issue a minute ago.
For me this is a very clear indicator that it's a software issue, not hardware.
Now I'm curious which versions of the phone modified for carriers do not have this issue
Best of luck to all