Wpf - RaisePropertyChanged Doesn't Update Binding

I've come across some strange behavior. I have a control that binds to a property like so:

<HyperlinkButton x:Name="ProjectBeheerLink"
Visibility="{Binding IsBeheerder, Converter={StaticResource VisibilityConverter}}"/>

In my viewmodel I have the property implemented like this:

Public ReadOnly Property IsBeheerder As Boolean
Get
Return iwtApp.AllMyFunctieRollen.Any(Function(x) x.Rol.Equals(Constants.RoleBeheerder))
End Get
End Property

Then when I raise my PropertyChanged event in my callback method

Private Sub GetMyPersonCompleted(ByVal lo As LoadOperation(Of FunctieRol))
'Init FunctieRollen ect. ...
RaisePropertyChanged(Function() Me.IsBeheerder)
End Sub

my binding does not update. However! If I implement a backend field like this:

_isBeheerder = iwtApp.AllMyFunctieRollen.Any(Function(x) x.Rol.Equals(Constants.RoleBeheerder))
RaisePropertyChanged(Function() Me.IsBeheerder)

And change my property like this:

Public ReadOnly Property IsBeheerder As Boolean
Get
Return _isBeheerder
End Get
End Property

Everything works fine...