Clear initial exposure value with zero. Seems to work fine for all tested platforms, and eliminates the black screen flashes on Metal without requiring any platform-specific hacks.

Assets/Scripts/Core/Fsr2Context.cs

@@ -204,7 +204,7 @@ namespace FidelityFX
                 
                 // Auto exposure always used to track luma changes in locking logic
                 commandBuffer.SetRenderTarget(_resources.AutoExposure);
-                commandBuffer.ClearRenderTarget(false, true, SystemInfo.graphicsDeviceType == GraphicsDeviceType.Metal ? Color.clear : new Color(-1f, 1e8f, 0f, 0f));
+                commandBuffer.ClearRenderTarget(false, true, new Color(0f, 1e8f, 0f, 0f));
 
                 // Reset atomic counter to 0
                 commandBuffer.SetRenderTarget(_resources.SpdAtomicCounter);