The documentation has an excellent explanation:
Differs from a normal ThreadPool work item in that ASP.NET can keep
track of how many work items registered through this API are currently
running, and the ASP.NET runtime will try to delay AppDomain shutdown
until these work items have finished executing. This API cannot be
called outside of an ASP.NET-managed AppDomain. The provided
CancellationToken will be signaled when the application is shutting
down.
Task.Factory.StartNew does not register work with the ASP.NET runtime at all. You’re running your code for 10 minutes, that makes no difference. IIS recycle happens at particular times which are preset in IIS. If you really want to test whats going on, you can attempt to force a recycle.