Why would an assert on a unit test with HostType("Moles") pass when run individually, but fail when run with a group of tests?

Question

I recently got aboard the Pex & Moles bandwagon in order to test some logic with many elements that are static, non-virtual, sealed, etc. Recently, I've begun to see behavior I can't explain from a few of the tests.

A couple of the methods for an interface I stubbed out return void, so I set the stubs to delegates that update boolean variables to indicate that they've been called. Here's what I'm doing:

[TestMethod]
[HostType("Moles")]
    public void UnitTestX()
    {
        bool disposeCalled = false;
        bool getCalled = false;

        ClassX classX = new ClassX();
        var data = new SIClassXData
                       {
                           Dispose = () => disposeCalled = true,
                           Get = () => getCalled = true,
                           ClassXGet = () => classX
                       };

        MDataLayerFactory.CreateDataLayerObject(() => (IClassXData)data);

        Assert.IsTrue(disposeCalled);
        Assert.IsTrue(getCalled);
    }

For whatever reason, the asserts above succeed if I run this test alone. But if I run the test along with every other test in the assembly (using Visual Studio's "Run All Tests in Solution" capability), the first assert fails.

I'd like to know why this occurs, and what I need to change to resolve the issue.