New Post: PropertyGrid ComboBox and Office themes

The behavior on my side was working properly by modifying the "CustomItemsSource.xaml" file from the "Providing an ItemsSource" sample:

<sample:DemoView x:Class="Samples.Modules.PropertyGrid.Views.CustomItemsSource"
                 xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                 xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                 xmlns:sample="clr-namespace:Samples.Infrastructure.Controls;assembly=Samples.Infrastructure"
                 xmlns:xctk="http://schemas.xceed.com/wpf/xaml/toolkit"
                 xmlns:xctt="http://schemas.xceed.com/wpf/xaml/toolkit/themes"
                 xmlns:xcpt="http://schemas.xceed.com/wpf/xaml/themes"
                 Title="Custom ItemsSource"
                 Description="Sometimes it is desirable to want to provide a collection of values represented by a ComboBox for a given property.  The PropertyGrid supports this scenario by create a class that implements IItemsSource and decorating your property with the ItemsSourceAttribute.">
   <StackPanel Margin="10">
      <StackPanel.Resources>
         <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
               <xctt:Office2007BlueResourceDictionary StyleUsageMode="Implicit" />
               <xcpt:Office2007BlueResourceDictionary StyleUsageMode="Implicit"/>
            </ResourceDictionary.MergedDictionaries>
         </ResourceDictionary>
      </StackPanel.Resources>
      <TextBlock Text="Providing a Custom ItemsSource:" Style="{StaticResource Header}" />
      <TextBlock Text="This example uses a collection of Doubles to represent the available options for the WritingFontSize property." TextWrapping="Wrap" />
      <xctk:PropertyGrid x:Name="_propertyGrid" Width="450" Margin="10" />
   </StackPanel>
</sample:DemoView>

Since you seems to be a "Plus" edition subscriber (or trial) I would recommend to contact our support team with a sample app that reproduce your problem so we can help you out.