Leonhard is referencing to the netgen.occ interface. I’d also advise to move to this more powerful interface to the opencascade geometry kernel.
However in the old csg kernel there are some caveats with boundary conditions: probably there is the same plane somewhere else defined in your geometry without the bc name. Csg then unifies them. To name the boundary save the plane in Python (for example as pl1) and then when adding to the Geo use bcmod:
geo2.Add(test, bcmod=[(pl1, "plane")])
This behavior is one of the very good reasons to move to occ… 
Best Christopher