The text box bottom border on UWP application is not visible

In short, you need to make a custom style for your TextBox.

The steps: Go to 'Document Outline -> Right click your TextBox -> Edit Template -> Edit a Copy.'

Then, find the <Border x:Name="BorderElement"> element in this style and set its MinHeight that you want.

<Page.Resources>
    <Style x:Key="TextBoxStyle1" TargetType="TextBox">
        ...
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="TextBox">
                    <Grid>
                        ...
                        <ContentPresenter x:Name="HeaderContentPresenter" ContentTemplate="{TemplateBinding HeaderTemplate}" Content="{TemplateBinding Header}" Grid.ColumnSpan="2" Grid.Column="0" FontWeight="Normal" Foreground="{ThemeResource TextControlHeaderForeground}" Margin="{StaticResource TextBoxTopHeaderMargin}" Grid.Row="0" TextWrapping="Wrap" VerticalAlignment="Top" Visibility="Collapsed" x:DeferLoadStrategy="Lazy" />
                        <Border x:Name="BorderElement" MinHeight="25" Background="{TemplateBinding Background}" BorderThickness="{TemplateBinding BorderThickness}" BorderBrush="{TemplateBinding BorderBrush}" CornerRadius="{TemplateBinding CornerRadius}" Grid.ColumnSpan="2" Grid.Column="0" Control.IsTemplateFocusTarget="True"  MinWidth="{ThemeResource TextControlThemeMinWidth}" Grid.RowSpan="1" Grid.Row="1" />
                        <ScrollViewer x:Name="ContentElement" AutomationProperties.AccessibilityView="Raw" Grid.Column="0" HorizontalScrollBarVisibility="{TemplateBinding ScrollViewer.HorizontalScrollBarVisibility}" HorizontalScrollMode="{TemplateBinding ScrollViewer.HorizontalScrollMode}" IsDeferredScrollingEnabled="{TemplateBinding ScrollViewer.IsDeferredScrollingEnabled}" IsHorizontalRailEnabled="{TemplateBinding ScrollViewer.IsHorizontalRailEnabled}" IsTabStop="False" IsVerticalRailEnabled="{TemplateBinding ScrollViewer.IsVerticalRailEnabled}" Margin="{TemplateBinding BorderThickness}" Padding="{TemplateBinding Padding}" Grid.Row="1" VerticalScrollMode="{TemplateBinding ScrollViewer.VerticalScrollMode}" VerticalScrollBarVisibility="{TemplateBinding ScrollViewer.VerticalScrollBarVisibility}" ZoomMode="Disabled" />
                        <TextBlock x:Name="PlaceholderTextContentPresenter" Grid.ColumnSpan="2" Grid.Column="0" Foreground="{Binding PlaceholderForeground, RelativeSource={RelativeSource Mode=TemplatedParent}, TargetNullValue={ThemeResource TextControlPlaceholderForeground}}" IsHitTestVisible="False" Margin="{TemplateBinding BorderThickness}" Padding="{TemplateBinding Padding}" Grid.Row="1" Text="{TemplateBinding PlaceholderText}" TextWrapping="{TemplateBinding TextWrapping}" TextAlignment="{TemplateBinding TextAlignment}" />
                        <Button x:Name="DeleteButton" AutomationProperties.AccessibilityView="Raw" BorderThickness="{TemplateBinding BorderThickness}" Grid.Column="1" FontSize="{TemplateBinding FontSize}" IsTabStop="False" MinWidth="34" Margin="{ThemeResource HelperButtonThemePadding}" Grid.Row="1" Style="{StaticResource DeleteButtonStyle}" VerticalAlignment="Stretch" Visibility="Collapsed" />
                        <ContentPresenter x:Name="DescriptionPresenter" AutomationProperties.AccessibilityView="Raw" Content="{TemplateBinding Description}" Grid.ColumnSpan="2" Grid.Column="0" Foreground="{ThemeResource SystemControlDescriptionTextForegroundBrush}" Grid.Row="2" x:Load="False" />
                    </Grid>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
</Page.Resources>
<Grid>
    <TextBox Style="{StaticResource TextBoxStyle1}" Height="25"></TextBox>
</Grid>

Please note that I only change the MinHeight for it, if you input in it, you will find that Its text display is incomplete. If you want to make it look better. You need to do more customization.

Please read Tutorial: Create custom styles and Control templates for more information.