Visual Studio 2017 Codelens not showing test connection when a method implements interface

Question

I have an issue with codelens when a class implements an interface, the 1/1 passing is not showing.

E.g. with this implementation:

public interface IMyInterface
{
   string GetString();
}

public class MyClass : IMyInterface
{
   public string GetString() 
   {
      return "A string";
   }
}

And a test:

[TestMethod]
public void MyTest() 
{
  IMyInterface mc = new MyClass()
  var str = mc.GetString();
  Assert.AreEqual(str, "A string");
}

If I remove the interface from the class, or comment out the string GetString(); from the interface, then 1/1 passing shows. But if it's there it's missing and the call from the test method is added to the reference in codelens

Any suggestions how to come around this?