Got the hirarchy shown in attached tree.png, defined by the following declaration

<Style x:Key="VirtualList" TargetType="{x:Type ItemsControl}" BasedOn="{StaticResource {x:Type ItemsControl}}">
    <Setter Property="OverridesDefaultStyle" Value="False"/>
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate>
                <Border
                    BorderThickness="{TemplateBinding Border.BorderThickness}"
                    Padding="{TemplateBinding Control.Padding}"
                    BorderBrush="{TemplateBinding Border.BorderBrush}"
                    Background="{TemplateBinding Panel.Background}">
                    <ScrollViewer x:Name="PART_Scroll"
                        Padding="{TemplateBinding Control.Padding}"
                        Focusable="False">
                        <ItemsPresenter />
                    </ScrollViewer>
                </Border>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
    <Setter Property="ItemsPanel">
        <Setter.Value>
            <ItemsPanelTemplate>
                <VirtualizingStackPanel/>
            </ItemsPanelTemplate>
        </Setter.Value>
    </Setter>
    <Setter Property="ScrollViewer.CanContentScroll" Value="True"/>
    <Setter Property="VirtualizingPanel.IsVirtualizing" Value="True"/>
    <!-- Recycling would be better, but Noesis appears to have a data race in the bindings -->
    <Setter Property="VirtualizingPanel.VirtualizationMode" Value="Recycling"/>
    <Setter Property="VirtualizingPanel.ScrollUnit" Value="Pixel"/>
</Style>

<DataTemplate x:Key="GnomeScheduleButtonTemplate">
    <Button x:Name="Sched_Button" 
        Content="{Binding Activity}"
        Style="{StaticResource GnomeScheduleButtonStyle}"
        Command="{Binding CmdSetHour}" />
</DataTemplate>

<DataTemplate x:Key="PopGnomeSchedRowTemplate">
    <ItemsControl  ItemsSource="{Binding Schedule}" ItemTemplate="{StaticResource GnomeScheduleButtonTemplate}" Height="35">
        <ItemsControl.ItemsPanel>
            <ItemsPanelTemplate>
                <StackPanel Orientation="Horizontal"/>
            </ItemsPanelTemplate>
        </ItemsControl.ItemsPanel>
    </ItemsControl>
</DataTemplate>

     <ItemsControl x:Name="SVActivities"
         ItemsSource="{Binding Schedules}" ItemTemplate="{StaticResource PopGnomeSchedRowTemplate}"
         Grid.Row="1" Grid.Column="1"  Style="{StaticResource VirtualList}"
         ScrollViewer.VerticalScrollBarVisibility="Auto">
     </ItemsControl>

The issue occurs on the highlighted element. After the element has been recycled, the inherited TextElement.FontFamily for the inner (not virtualized) ContentPresenter node becomes empty.
When this happens, it impacts all children of the inner ItemsControl. The inner ItemsControl itself still has a sensible value for FontFamily (also inherited) in that state, according to the inspector, but inheritence from it is broken.

Setting and clearing a local value for TextElement.FontFamily on the broken ContentPresenter node restores the correctly inherited values, both according to inspector and visually confirmed.