ReactiveCommand in WPF/ReactiveUI app blocks UI thread - wpf

Question

I am quite a beginner with ReactiveUI and have a strange behavior with a ReactiveCommand.
I want to query data from a database that currently does not support
asynchronous operations. Since we want to exchange the database in the
future with an asynchronous interface I want to write everything as
if the database already would allow async operations. As far as I understand
that would mean that I wrap my database calls at the lowest level in
a Task.
I have a button which is bound to a ReactiveCommand and the command
starts the database query. While the query lasts I want to show some
sort of animation.
The problem is that whatever I tried, the query blocks my UI thread.
Here is part of my code:
public ReactiveCommand<Unit, Unit> StartExportCommand { get; }
//The constructor of my view model
public ExportDataViewModel(IDataRepository dr)
{
this.dr = dr;
//...
StartExportCommand = ReactiveCommand.CreateFromTask(() => StartExport());
//...
}
private async Task StartExport()
{
try
{
Status = "Querying data from database...";
//Interestingly without this call the Status message would not even be shown!
//The delay seems to give the system the opportunity to at least update the
//label in the UI that is bound to "Status".
await Task.Delay(100);
//### This is the call that blocks the UI thread for several seconds ###
var result = await dr.GetValues();
//do something with result...
Status = "Successfully completed";
}
catch(Exception ex)
{
Status = "Failed!";
//do whatever else is necessary
}
}
//This is the GetValues method of the implementation of the IDataRepository.
//The dictionary maps measured values to measuring points but that should not matter here.
//ValuesDto is just some container for the values.
public Task<IDictionary<int, ValuesDto>> GetValues()
{
//...
return Task<IDictionary<int, ValuesDto>>.Factory.StartNew(() =>
{
//### here is where the blocking calls to the database
//### specific APIs take place
return result;
}, TaskCreationOptions.LongRunning);
}
I don't understand why this code is blocking the UI thread although I am wrapping
the long running query in a Task.
Is there something wrong with this pattern or should I go another way with Observables?
Edit 1
I am aware of the fact that async != threads. I thought, however, that Task with the TaskCreationOptions.LongRunning would make the blocking code run on a thread pool thread.
Edit 2
As recommended by Andy I set a breakpoint inside my task and had a look into the Debug Threads window. It tells me that Task is running on a worker thread. Still my UI is blocking.