Adding further to do this:
In 4.26, this seems to work completely as expected.
With r.AllowOcclusionQueries=0, you can see that statically occluded primitives are still being occluded correct (indicated by the dark red debug boxes on the right). With occlusion queries enabled, you can see that they both work in tandem
So clearly, something is broken at least in 5.6+.