Forward Rendering causes Volumetric Material to not render against background

Regression: Tested in UE5/Release-5.0, and the issue is reproduceable.

This issue appears to occur when the SkySphere is deleted from the level, as leaving the SkySphere in the level causes the Volumetric Material to render the same as if there was geometry behind it.

1. Create an Empty Template Project
2. Enable Forward Rendering and restart
3. Create a new material
4. Place a cube in the level and assign the material to the cube
5. In the material, change its Material Domain to Volume and Blend Mode to Additive
6. Create a Constant Vector 3 and make it any color
7. Connect the Constant Vector 3 to the Albedo
8. Create a constant with a value of 0 and connect it to the Emissive Color
9. Create a constant with a value of 1 and connect it to the Extinction 
10. Apply the material and wait for shaders to compile
11. Delete the SkySphere and Volumetric Clouds in the level
12. Enable Volumetric Fog in the Exponential Height Fog
13. Place a cube behind the first cube, so that you can see both the sky and cube through the cube with the Volumetric Material

Actual Result: The Volumetric Material does not render against the sky with Forward Shading on, it renders properly with Forward Shading off.

Expected Result: The Volumetric Material renders the same between Forward Shading enabled and disabled.