DefaultStyleKey style not found in inner MergedDictionaries by Andrej Burger

Situation:
Window.Resources.MergedDictionaries contains AdministrationResources.xaml (ResourceDictionary)
AdministrationResources.xaml contains GlobalResources.xaml in MergedDictionaries

GlobalResource.xaml contains style:
<Style TargetType="{x:Type Button}" BasedOn="{StaticResource {x:Type Button}}">
    ...
</Style>

Window contains control:
<Button x:Name="OKButton" Content="OK" />

What I found out:
Style in GlobalResources is applied in VS designer, but not in runtime (default WPF button style is used).
If style is defined in Window.xaml or in AdministrationResources.xaml it works good.
If I set buttons style explicitly it works good. Even if I set it to the value of DefaultStyleKey.
<Button x:Name="OKButton" Content="OK" Style="{StaticResource {x:Type Button}}" />

Default style key is always set correctly to typeof(Button).
Window.FindResource(typeof(Button)) works correctly.

It is working code from .NET 3.5 SP1, now it does not work in .NET 4.0 supplied with VS2010 RTM.

I think:
When looking for style defined by DefaultStyleKey (not set explicitly), only one level of merged dictionaries is searched.