tests fail when run together but pass individually

I'm getting strange results that I don't understand. Either of these unit tests will pass when run individually:

[Isolated, TestMethod, Owner("Mark Phillips"), TestCategory("Default Replenishment Information")]
public void UpdateDefaultReplenishmentByStockTypeId_CaseQuantityTooLow_ExpectValidationError()
{
//Arrange
string expectedError = "Case quantity must be 1 or greater.";

Isolate.NonPublic.WhenCalled<ProductReplenishmentData>("StockTypeIdIsValid").IgnoreCall();

ProductReplenishmentDefaults defaults = ProductReplenishmentDefaults.NewReplenishmentItemDefaults();
ProductReplenishmentData productReplenishmentData = ProductReplenishmentData.NewReplenishmentItemForDefaultsList(defaults, true);

//Act
productReplenishmentData.CaseQuantity = 0;

//Assert
Assert.IsFalse(productReplenishmentData.IsValid);
Assert.AreEqual<string>(expectedError, productReplenishmentData.BrokenRulesCollection.ToString().StripErrorCode());
}

[Isolated, TestMethod, Owner("Mark Phillips"), TestCategory("Default Replenishment Information")]
public void UpdateDefaultReplenishmentByStockTypeId_CaseQuantityEdgeValue_ExpectValid()
{
//Arrange
Isolate.NonPublic.WhenCalled<ProductReplenishmentData>("StockTypeIdIsValid").IgnoreCall();

ProductReplenishmentDefaults defaults = ProductReplenishmentDefaults.NewReplenishmentItemDefaults();
ProductReplenishmentData productReplenishmentData = ProductReplenishmentData.NewReplenishmentItemForDefaultsList(defaults, true);

//Act
productReplenishmentData.CaseQuantity = 1;

//Assert
Assert.IsTrue(productReplenishmentData.IsValid);
}

But when run together the second one fails with System.InvalidOperationException: Cannot register property CaseQuantity after containing type (ProductReplenishmentData) has been instantiated.

Both methods have the Isolated attribute, so why would the results be different when run individually versus together?

I am using version 6.0.10.0.

Thanks,
Mark