Style within DataTemplate is only being applied to the last item in ItemsControl?

In the XAML below, I have an ItemsControl that has three DataObjects.
I use a DataTemplate to display DataObjects as Buttons with an "X" on them.
The Button uses a Style to set its Content.

If the Setter.Value is "X", everything works great!
However, if I change the Setter.Value to a TextBlock whose TextProperty is "X", the X only appears on the last Button (the third DataObject) and the first two Buttons are empty.

Is this a bug, or can anybody explain why this happens?

Note 1) This is a contrived example to isolate the problem being encountered.
Note 2) I've put both Setter.Value options in the code so you can reproduce both the successful and unsuccessful cases just by having one of them commented out.
Note 3) It appears, this problem is specific to Setters for the 'Content' property. If I use a Setter for the Background property, it correctly applies to all of the DataObjects.

<Grid>
    <Grid.Resources>
        <Style x:Key="myButtonStyle" TargetType="{x:Type Button}">
            <Setter Property="Content">
                <!--<Setter.Value>X</Setter.Value>-->
                <Setter.Value><TextBlock Text="X" /></Setter.Value>
            </Setter>
            <Setter Property="Background">
                <Setter.Value>
                    <SolidColorBrush Color="Red" />
                </Setter.Value>
            </Setter>
        </Style>
    </Grid.Resources>
    <ItemsControl>
        <ItemsControl.ItemTemplate>
            <DataTemplate DataType="{x:Type DataObject}">
                <Button Height="24" Width="24" Style="{StaticResource myButtonStyle}" />
            </DataTemplate>
        </ItemsControl.ItemTemplate>
        <ItemsControl.Items>
            <DataObject />
            <DataObject /