Forward Rendering MSAA causes black outlines of meshes when partially occluding Sky Atmosphere

Steps to reproduce the outline issue:

1. Create a project.
2. Open the project settings, then make the following changes:
   1. Rendering > Default Settings > Anti-Aliasing Method: MSAA
   2. Forward Renderer > Forward Shading: Checked
3. In the level, do the following:
   1. Remove the sky sphere and the atmospheric fog.
   2. Insert a SkyAtmosphere object.
   3. Insert a cube or some other mesh.
   4. Move the viewpoint so that the mesh obstructs the sun (doesn't matter if you're in PIE or not). You should see a black outline around the mesh.