I had the same flashing under DX12 and tried everything in this thread with no success. For me, I was able to trace it back to any “MXAO” shaders. When one of them is active, I get the flashes. When I deactivate them, the flashes disappear. BTW, I did try moving it/them to the top of the list of options as the text advises, but the flashes continued and didn’t stop until I deactivated it.
If you’re still having the problem and you use Reshade, give that a try. As a secondary option, if you’re using Reshade, deactivate it altogether and see if that works.