DDM_AllOpaque means everything. This is hard requirement and is used for rendering algorithms that require a complete depth prepass. DDM_AllOccluders is for respecting the occluder flag.
The licensee is stating that static mesh elements are always drawn as occluders. In FStaticMeshSceneProxy::GetMeshElement, the mesh batch doesn't seem to be taking in the bUseAsOccluder flag value.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-43919 in the post.