A strange issue is occurring with the Beauty (Combined) Pass when Render Layers are used with Motion Blur.
Version: 2018.1.3 / Octane Engine 17.9
If I enable both a normal Beauty Pass and one or more of the Render Layer Passes with Motion Blur, then the Beauty Pass does not render correctly. It starts to render but is then subsequently over written by whatever layer pass is active, so that the layer pass effectively renders twice! If Motion Blur is disabled, then the Beauty Pass renders normally along with the other render passes (i.e. it is no longer overwritten by them).
This happens with either Subframe or Internal Motion Blur enabled ( Internal MB has never actually worked for me but that is an issue for another post).