No exception when interacting with UI element in background thread

We have an event aggregator (similar to Prism) that is used in our codebase. It allows consumers to subscribe a handler that returns a Task so asynchronous work can be done without using async void. I found that when you do await aggregator.PublishAsync(new SomeEvent()) you end up waiting for all handlers to execute before the task is done. If you have an expensive handler that takes 2 seconds then that publish will take ~2 seconds.

It is now allowed for consumers to subscribe an Action<TEvent> or Func<TEvent, Task> to the aggregator. You can also, when subscribing, say whether or not you want that handler to be executed on the UI thread or not. We wanted to give the option to wait for all handlers or Fire/Forget by returning the Task. We also wanted to make sure if you don't await the publish that you aren't waiting the ~2 seconds or however long. So this is what we came up with:

public Task PublishAsync<T>(T @event) where T : IEvent
{
    var subscriptions = _subscriptions
        .Where(kvp => kvp.Value.EventName == typeof(T).Name)
        .Select(kvp => kvp.Value)
        .ToList();

    // Task.Run is used so the work is done with an available thread in the thread pool
    Task backgroundTask = Task.Run(async () =>
    {
        var backgroundTasks = subscriptions
            .Where(s => !s.ExecuteOnUiThread)
            .Select(s => s.Handler.Invoke(@event));

        // _uiContext = SynchronizationContext.Current happens on construction (and is constructed in the main UI thread)
        var uiThreadTasks = subscriptions
            .Where(s => s.ExecuteOnUiThread)
            .Select(s => { _uiContext.Post(obj => s.Handler.Invoke((T)obj), @event); return Task.CompletedTask; });

        await Task.WhenAll(backgroundTasks.Concat(uiThreadTasks)).ConfigureAwait(false);
    });

    return backgroundTask;
}

We have a view model that is subscribing to an event. The handler updates a property that is bound to a Label's Text property. If we await the PublishAsync call and that handler says not to use the UI thread I'll get a "cross thread" exception like I expect. If I fire/forget and do something like _ = _aggregator.PublishAsync(...); the property is assigned to and everything works (even though I'm NOT on the main UI thread). I'm at a loss. How is the below screenshot possible? Executing line 41 should've thrown an exception.

Because I was doing a fire/forget publish of the event everything executed fast enough for the window to be closed by the view model before the handler was called. By the time the handler was called the connection with the UI elements (through binding) was severed and setting that property was fine since it wasn't going ahead and interacting with UI elements.

When I removed the code that closed the window and interacted with the event aggregator as designed I was getting the cross-thread exceptions as expected regardless of fire/forget or async/await. Subscribing the handler that says to execute it on the UI thread was also working.