I have exactly the same issue, have been trying to fix it for a few months actually.
I checked all open programs as well, I couldn't find any that was causing it.
System: Fully updated Budgie.
I also observed 2 different behaviours:
- screen fading to black and then immediately going to white
- screen turning off, and after a few seconds turning on but with an image of lock screen. If you then moved your mouse it would show desktop
I have been getting mostly the 2nd behaviour, but after I played around with setting of screensaver in dconf editor I now get the first one. Still no idea what's causing it.