Several generations of betas avoid screen turning off (energy save) after x minutes
The screen goes off for a few seconds after the set up count down but immediately turns on again.
This issue is only solved by exiting f.lux - I researched the web and tried several other "solutions" before ...
Can you double-check you have not enabled any home automation features (Hue, etc.) - if these are on, it will prevent sleeping. We should probably make an option where this isn't true.