Ubuntu Budgie 18.04 - ‘Night Light’ flickers occasionally
Solution 1:
I've had that same problem and was able to find the source of the problem.
I had another program installed before 18.04 called "Red Shift" that was conflicting with the "Night light" feature. I removed that program and the issue was fixed since then.
You should check if you do not have another program that tries to manage the screen at the same time.
Solution 2:
In my case it was flux that I was running , along with Night Light in GNOME.
I removed the conflicting service, and didnt even need to reboot - Night Light started working as expected again.