There's so many people having similar issues with Freesync and none getting any answers. I myself have identical problem with brightness flicker/strobing in games and menus/loading screens after enabling Freesync in Radeon Settings. There's only one game that doesn't flicker for me - Doom. It uses Vulkan API. I'm not sure whether it has anything to do flicker-free experience but it's the only thing sets it apart other games I play (mainly BF1 at the moment). So if I had to guess, I'd say it's not hardware we need to blame.
I use exactly the same Agon monitor and Saphire's Fury card. Also I bought a VESA certified cable as suggested by some users. I mainly went for AMDs Freesync setup (card + monitor) as I couldn't justify paying a £100 more for G-Sync. Maybe that wasn't the best choice... Anyway, It's been 6 months since my purchase and I still can't use Freesync (flicker-free).
I've read many posts on different forums and they all suggest same methods of ''dealing'' with this issue. None of them resolve it though. I'm pretty sure AMD is well aware of the problem but since they don't have any solution, they treat anyone who asks about it as if they were a one-off, isolated case. I hope it gets resolved one day. For the time being, making your game run at steady 80-100 FPS makes the experience pretty tear-free (at least most of the time).
Was Google'ing the solution to same problem and I found this thread.
Reading pcpme's first post was like it was written by myself... even the configuration is same!
AOC AG271QX 27.0" 2560x1440 144Hz Monitor
Ryzen 5 1600
XFX Radeon RX 480 8GB GTR Video Card
Gigabyte GA-AB350M-GAMING 3
Corsair Vengance LPX 16GB (2 x 8GB) DDR4-3200 Memory
I tried going back to older driver: 17.4.2, but similar result, still flickers.... like the panel PWM is not working or something when FreeSync is enabled... When I switch FreeSync OFF then the flickering stops. But for me the flickering is also in the movies I watch or in the TV series!
For F*U*C*K sake AMD, fix the problem! It is not isolated case.