I know we support app authentication for tasks, but it actually doesn't make much sense in this case. Users are connected to other users through shared spaces, that how we determine if one user can see and e.g. assign a task to another user.
But with app authentication you are actually "signed in" as the app, and the app can't see any users at all. For this reason it is not possible to assign tasks to anyone with app authentication. Instead, you should authenticate as a user when creating tasks through the API.
We do see the value of being able to work with tasks using app authentication, and will look further into the possibilities of making a sensible permission model for apps in regards to tasks.