18.0.325.
The SOP Modify LOP is unable to read in geometry from a Sublayer or Load Layer LOP. Is this a bug or am I missing something?
Reference LOP -> SOP Modify LOP works fine.

The SOP Modify uses the lopinputprim expression by default to choose which input prims to import into the SOP context. The Reference LOP sets its “last modified prim” to the root of the reference. The Sublayer and Load Layer LOPs don't set a “last modified prim” because there is no clear choice when adding a sublayer this way can add any number of root primitives. So when using those LOPs as inputs to the SOP Modify, you need to change the “Primitives” parameter to specify which parts of the scene to bring into SOPs.