The brightness has no impact.
At normal Windows use:
When the screen is set to 300Hz there is no flickering. If I set the refresh rate to 60Hz I immediately can see flickering.
During the reset to factory settings, flickering was also present during Windows installation process.
Flickering at around 60FPS. If the game runs at +100FPS no flickering can be seen.
I tried yesterday with FreeSync disabled in AMD Software and VSync disabled in game and that seems to eliminate the flickering in games because the screen then runs at higher/300 Hz!?
The problem with the screen seems to be at lower refresh rates.
I now don't know if I should return the laptop or if it is perhaps a software issue?
Are you using Windows 10 pro and Hyper-V ? I also had exactly the same problem with the random flicker of the screen and I solved it once and for all.