MSTest - Moq setup with a list parameter returns empty list

I recreated a Minimal, Complete, and Verifiable example of a test for your scenario and was able to get it to pass. Take a look at the following

[TestClass]
public class MoqSetupWithListParameterTests : MiscUnitTests {
    //m_Items is a static list I filled in the Class Initialize, and is not empty. 
    static IEnumerable<Item> m_Items = Enumerable.Range(1, 10).Select(i => new Item());
    //The mock is also static. 
    static Mock<IWebRequestManager> WebRequestManagerMock = new Mock<IWebRequestManager>();

    [TestMethod]
    public void Items_Count_Should_Equal_Orders_Count() {
        //Arrange
        var expected = 3;
        //Orders is also not empty when I debug the test.
        var m_Orders = Enumerable.Range(1, expected).Select(i => new Order());
        WebRequestManagerMock
            .Setup(x => x.GetItemsAsync(It.IsAny<IEnumerable<Order>>()))
            .Returns<IEnumerable<Order>>(orders => Task.FromResult(m_Items.Take(orders.Count())));

        var sut = WebRequestManagerMock.Object;

        //Act
        //When I call GetItemsAsync I get expected count.
        var actual = sut.GetItemsAsync(m_Orders).Result;

        //Assert
        Assert.AreEqual(expected, actual.Count());
    }

    public interface IWebRequestManager {
        Task<IEnumerable<Item>> GetItemsAsync(IEnumerable<Order> enumerable);
    }

    public class Order { }
    public class Item { }
}