Indeed, the "Dynamic Contrast" option was active and, after it, the problem was gone.

I'm not sure if this occurs only with v4 because I don't remember if this monitor option was active before. I use f.lux since 2014 and I have this monitor since January 2015, and this never occurred until now.

Maybe, because of the nature of this option (as you pointed out), together with the dynamic nature of f.lux, which is reacting to the luminosity of the program active, both progressively interfere in each other: one adjust, then the other too, recursively.

Maybe some analisys over the periodicity with f.lux is changing luminosity, to prevent this cycle from occurring, i don't know, it's only a suggestion.

Can you check the Philips on screen menu and see if there is any "dynamic" or "adaptive" kind of setting? These things can be very aggressive depending on the kind of content.