This is a bug, Mock.Vertify verifies only the instance calls and not the base class calls.
As a workaround, you can use the VerifyMocks attribute:
public void Test()
Mock<DerivedClass> mock = MockManager.Mock<DerivedClass>();
// mock only BaseClass.SomeMethod when called from a DerivedClass Type.
DerivedClass d = new DerivedClass();