WPF - MenuItem missing Icon/Image

You are setting Icon property to an Image control in Style. Now, only one copy of Style is created and thus, only one copy of Image is created. Now, any control can have only one parent at a time. So, when it is assigned to last MenuItem, it is removed from previous MenuItem controls. To fix this, use Templates.

Instead of setting Header property, set HeaderTemplate:

            <Setter Property="HeaderTemplate">
                <Setter.Value>
                    <DataTemplate>
                        <Grid>
                            <Grid.ColumnDefinitions>
                                <ColumnDefinition Width="Auto" />
                                <ColumnDefinition Width="*" />
                            </Grid.ColumnDefinitions>
                            <Image Grid.Column="0"
                                   Source="{Binding Path=IconPath}" />
                            <TextBlock Grid.Column="1"
                                       Text="{Binding DisplayName}" />
                        </Grid>
                    </DataTemplate>
                </Setter.Value>
            </Setter>

I'm not sure of what properties are exposed by the control toolkit you are using. But, I'm sure they must have a template property.

After doing this, you don't need to set Icon property in style.

I successfully use the following entries in a ResourceDictionary:

<!-- Define non-shared image to avoid loss of menu icons -->
<Image x:Key="MenuIconImage" Height="16" Width="16" x:Shared="false">
    <Image.Source>
        <DrawingImage Drawing="{Binding Icon}" />
    </Image.Source>
</Image>

<Style TargetType="{x:Type MenuItem}" BasedOn="{StaticResource {x:Type MenuItem}}">
    <Setter Property="Header" Value="{Binding DisplayName />
    <Setter Property="Icon" Value="{StaticResource MenuIconImage}" />
</Style>