Please correct me if I'm wrong but from my understanding:

You took the Subterranean Graverobber edge.
That makes subterranean areas qualify as graveyards for your spells (yay).
You EXPECT that to mean you can summon in subterranean areas.

The third part is the problem.
The edge does not make areas you can't summon in into ones you can summon in. It just makes those rooms into graveyards.

If I misunderstand what you're saying, respond and tear me apart!