Auto correction behaviour on XAML bound datetime objects since .NET 4.0?

While bringing an application from .NET 3.5 to .NET 4.0 I've run into this peculiar issue.

(culture is nl-BE)

I bind a TextBox like this (in XAML) to a DateTime value with an UpdateSourceTrigger on PropertyChanged (LostFocus works as expected but as-you-type validation is required):

<TextBox Height="23" Margin="146,0,105,97.04" Name="txb_Geboortedatum" VerticalAlignment="Bottom">
        <TextBox.Text>
            <Binding Path="Geboortedatum" StringFormat="d" 
                     UpdateSourceTrigger="PropertyChanged">
                <Binding.ValidationRules>
                    <ExceptionValidationRule />
                </Binding.ValidationRules>
            </Binding>
        </TextBox.Text>
</TextBox>

Now when the contents of this textbox is (for example) 10/12/2000 and I want to edit it to be 09/03/1981 some obnoxious auto-correction occurs when i put the cursor at the end of 2000 and start 'backspacing' away the year value (when only the first digit ('2') of '2000' is left the value automatically - including cursor jump - changes to 2002 again). Can I disable this auto-correction?

I can't seem to find what specifically introduced this behaviour. The same 'problem' also occurs with FormatString=c for currency values.

What I've tried so far:

1. Changing the FormatString to something more explicit like {0}{dd/MM/yyyy} (same problem: starts auto-correcting when there are 2 digits for year left).

2. Disabling the following snippet I've added to my App.xaml.cs:

   FrameworkElement.LanguageProperty.OverrideMetadata(
       typeof(FrameworkElement), 
       new FrameworkPropertyMetadata(XmlLanguage.GetLanguage(
           CultureInfo.CurrentCulture.IetfLanguageTag)));
   

The reasoning for this snippet to be included in the first place: have a look at this link.

Am i missing something obvious here? I can't reproduce this in 3.5. Do I really have to roll my own ValueConverters for getting this to work properly? That looks like a step back from StringFormat which was introduced in 3.5 sp 1.

Output from DateTimeFormatInfo.CurrentInfo.GetAllDateTimePatterns('d') does looks slightly different, nothing that would immediately explain the behaviour though (probably unrelated):

.NET 3.5        .NET 4.0

d/MM/yyyy       d/MM/yyyy
d/MM/yy         d/MM/yy
dd-MM-yy        dd-MM-yy
dd.MM.yy        dd.MM.yy
yyyy-MM-dd      dd.MMM.yyyy
                yyyy-MM-dd