AO Decals not working when using Lumen

When lumen enabled, and with static lighting disabled, we expect the Material AO to contribute to diffuse lighting occlusion. However AO from decals does not have any effect.

Open QAGame and enable lumen and disable static lighting in the renderer project settings.

Open the TM_Decals map and go to the SM_Base_Mesh_Decal_Static_in_Shadow area of the map.

Validate that MaterialAO is working by opening the M_Concrete material and changing it so that a constant 0 is passed into the AmbientOcclusion output. Apply the change and observe the visual darkening of the concrete block. Now revert the material change, so that the concrete block looks normal again.

Now edit the M_MeshDecal_Param material in the same way. Set a constant 0 to the AmbientOcclusion output and apply.

If AO decals are working we would expect the red decals on the concrete block to darken in the way that the whole block darkened earlier. Observe that it doesn't do this.