Moq lambda expression behaving unexpectedly

I have a unit test using MOQ that's behaving unexpectedly. I'm expecting the IsAuthorizedAsync method to always return true, but it's returning false. Here's a simplified version of the code that's adding the IsAuthorizedAsync method to my Mock Object.

mock.Setup(e => e.IsAuthorizedAsync(It.IsIn<string>(), It.IsAny<MyEvent>())).Returns(System.Threading.Tasks.Task.FromResult(true)).Verifiable();

// return the mock object
return mock.Object;

Here's code similar to the code that's using it:

bool isAuthorized = this.mockObject != null && await this.mockObject.IsAuthorizedAsync("abc123", myEvent).ConfigureAwait(false);

Like I said, it's returning false when it looks to me like it should always return true. Is there any way for me to step into the lambda expression code? Alternatively, is there any way for me to view what the actual lambda expression that's being used when I call this.mockObject.IsAuthorizedAsync? I suspect it's not what I think it is.

-Eric