Card doesn't display the header when in a ListBox

I have a Card in the DataTemplate for a ListBox and for some reason, the header doesn't show. I even added the "IsHeaderVisible=True". This is on Linux, if that matters. Am I missing something? Here is the code and a screenshot of the output. {Binding Conversion} creates the "Fahrenheit -> Celsius" in the first Card and "Fahrenheit -> Kelvin" in the second Card.

        <ListBox ItemsSource="{Binding Conversions}" Focusable="False">
            <ListBox.ItemTemplate>
                <DataTemplate>
                    <actipro:Card IsHeaderVisible="True">
                        <actipro:Card.Header>
                            <TextBlock Text="{Binding Value}" FontWeight="Bold"/>
                        </actipro:Card.Header>
                        <actipro:Card.Content>
                            <TextBlock Text="{Binding Conversion}" HorizontalAlignment="Center"/>
                        </actipro:Card.Content>
                    </actipro:Card>
                    <!-- <DockPanel> -->
                    <!--     <Label DockPanel.Dock="Left" FontSize="12" VerticalAlignment="Center" Content="{Binding Index}" /> -->
                    <!--     <Label DockPanel.Dock="Top" FontSize="20" HorizontalAlignment="Center" Content="{Binding Value}" /> -->
                    <!--     <Label DockPanel.Dock="Bottom" FontSize="14" HorizontalAlignment="Center" Content="{Binding Conversion}" /> -->
                    <!-- </DockPanel> -->
                </DataTemplate>
            </ListBox.ItemTemplate>
        </ListBox>