MSTest ignores test classes with non-test generic methods that have type constraints defined in a different assembly in .NET 4.5.1

I've been puzzling over this one for the past hour or so. I have an app and a set of unit tests that target Windows 8.1 / WinRT / .NET 4.5.1. All projects and the solution compile and link successfully, and the app runs successfully. The app contains a number of assemblies, two of which are described below.

Suppose assembly 1 contains the unit tests:

// assembly 1
using Microsoft.VisualStudio.TestPlatform.UnitTestFramework;
using NamespaceInSameAssembly;
using NamespaceInDifferentAssembly;

namespace Tests {
   [TestClass]
   public class DownloadTests {
      [TestMethod]
      public void Test1() {
         int x = 2;
         Assert.AreEqual(2, x);
      }

      private void HelperThatWorks1<T>() where T : IInterface1 { }
      private void HelperThatWorks2<T>() where T : IInterface2 { }
      private void HelperThatWorks3() {
         IInterface3 x = null;
      }
      //private void HelperThatDoesntWork<T>() where T : IInterface3 { }
   }
   public interface IInterface1 {}
}

namespace NamespaceInSameAssembly {
   public interface IInterface2 { }
}

And assembly 2 contains other, non-unit-test related stuff:

// assembly 2
namespace NamespaceInDifferentAssembly
{
   public interface IInterface3 { }
}

If I execute Test1() using the MSTest runner, it runs successfully and the test appears in the Unit Test Explorer window. If I then uncomment HelperThatDoesntWork() and execute the test, the test doesn't run, and MSTest removes it from the Unit Test Explorer window.

The only difference I can see is that IInterface3 is defined in a separate assembly, although it's not clear to me why this would matter for generic type constraints but not references of that type.

Any ideas why this might be happening?