AmbientOcclusion doesn't work properly on orthogonal projection camera

1. Add a camera to the scene.
2. Select the camera actor and set the Global Illumination method to None
3. Select the camera actor and set the Ambient Occlusion Intensity to 1.0
4. Change the View Mode from "Perspective" to "CameraActor" on the left top corner of the viewport.
5. Make sure you can see the ambient occlusion.
6. Select the camera actor and set the projection mode to "orthographic"
7. The ambient occlusion disappeared.

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