Looks like that for me:

<no location info>: warning: [8;;https://errors.haskell.org/messages/GHC-42258GHC-422588;;] [-Wunused-packages]

I figured it out after long debugging. I specified all the haskell dependencies using plan.json, so there should not be duplicated package instances.
However random is duplicated as you see in the log. It's because of operational package, which QuickCheck depends.
operational depends random in its executable component, not in its library component. So the generated plan.json doesn't include operational-to-random dependency.

However, cabal2nix of operational has random as its argument

{ mkDerivation, base, lib, mtl, random }:
mkDerivation {
  pname = "operational";
  version = "0.2.4.0";
  sha256 = "71581f7f89fce996079b19bf84ea473b47a3dab08fec9cc64266f3dff5e295c3";
  revision = "1";
  editedCabalFile = "1b5vjp87lh34lpp9i4mrwcmr6rs45r6azdamwinlhrxynn91n8ri";
  isLibrary = true;
  isExecutable = true;
  libraryHaskellDepends = [ base mtl ];
  executableHaskellDepends = [ base mtl random ];
  homepage = "http://wiki.haskell.org/Operational";
  description = "Implementation of difficult monads made easy with operational semantics";
  license = lib.licenses.bsd3;
  mainProgram = "operational-TicTacToe";
}