Not sure if it’s the same issue you’re having - I get flashes when using DX12 in MSFS, however, if I turn all the lights off in the plane the flashing disappears. I don’t see the issue when using DX11.
I managed to find some answers to my problem. It is the ASW causing trouble, but somehow turning ASW in the OTT did not help. So I had to use both the OTT and the Oculus debug tool (where I turned off the ASW).