Skylight raytrace shadow artifacts on flat plane

I have a scene with a SunSky actor and a flat plane with a material instance assigned from the Quixel master material. I noticed horizontal line artifacts that flash on the plane and I tracked it down to the Skylight Raytrace shadows. The skylight is stationary. If I set the plane y rotation to 0.01 so it is no longer perpendicular with the sky, the artifacts go away. If I assign some other shader such as M_GreyBall to the plane, the artifacts go away. However I can use the Quixel material on another non-flat object and the artifacts do not appear. If I assign the material to a cube, I can see the artifacts on the top side. I attached a video showing the artifacts. Is this expected behavior or a bug?

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