Why doesn't [FromQuery] work when unit testing a Razor page?

Question

I have this property in a PageModel:

[FromQuery(Name = "paypalPlanId")] public string? paypalPlanId { get; set; }

It works when calling the page within the application - the property is set to the value of the querystring parameter.

When I call it from a unit test using a mocked HttpContext, with a mocked HttpRequest containing the the same querystring, it does not work - the property is null.

Here is the calling part of the unit test:

// arrange
queryParameters.Add(Tuple.Create("paypalPlanId", "TESTPLAN1"));
var config = CoachWorkbench.Test.TestHelpers.TestHelperConfiguration.GetConfiguration();
HttpContext httpContext = CommonTestHttpContext();
PlanEditModel page = new PlanEditModel(config);
page.AddPageModelContextExtension(httpContext, errorList);

// Act
IActionResult result = await page.OnGetAsync();

I have hundreds of other unit tests using the same mocked HttpContext & HttpRequest code and they work fine. Testing specifically for the correct QueryString also works fine. I was expecting the [FromQuery] field to be populated with the query value.

I am assuming that either a) the [FromQuery] functionality uses some HttpRequest component that I am unaware of and have not mocked or b) the [FromQuery] functionality uses some middleware functionality that I am unaware of and that is bypassed by the unit test.