I noticed too once something gets culled from rendering it impacts GI, and if you stand in Directional Light shadow it seems to switch off GI. Though all seems a bit random.
And there is no AO from VXGI that I would notice, but using the viewport controls in the upper left I was able to confirm UE’s AO and HBAO+ do add up to each other (can switch them on and off there).
Further if I enable VXGI’s specular nothing happens and SSR’s aren’t disabled.