RichTextBlock hides its content when programmatically updated, twice

Question

While working on a small UWP application that displays rich text and updates portions of it based on model changes, I've stumbled upon a really strange RichTextBlock bug. I'd appreciate if someone could provide an insight on this RichTextBlock peculiarity or give an idea on fixing this.

Here is a simplified reproduction code and bug's use case:

• RichTextBlock with code-updated content works just fine until user clicks on it. After the click it displays one more change, but on the 2nd change the content becomes hidden.

• After that, if user selects something in the now invisible text or presses Ctr+A the RichTextBlock would re-display it's content, but on the 2nd text change after that the content becomes hidden again.

• What's curious, using the "Do Nothing" button makes content permanently appear, although clicking on the RichTextBlock once more makes it disappear again.

<Page
    x:Class="RichTextBlockTest.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">

    <Viewbox>
        <StackPanel Spacing="8" Margin="4">
            <RichTextBlock FontSize="24">
                <Paragraph>
                    <Run x:Name="timeRun">{ Time is Now }</Run>
                </Paragraph>
            </RichTextBlock>
            <Button HorizontalAlignment="Stretch">Do Nothing</Button>
        </StackPanel>
    </Viewbox>
</Page>

using System;
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;

namespace RichTextBlockTest
{
    public sealed partial class MainPage : Page
    {
        private DispatcherTimer timer =
            new DispatcherTimer { Interval = new TimeSpan(0, 0, 1) };

        public MainPage()
        {
            InitializeComponent();
            timer.Tick += (o, e) =>
            {
                timeRun.Text = DateTime.Now.ToString("HH:mm:ss");
            };
            timer.Start();
        }
    }
}

Besides the above, I've tried this scenario with binding, DependencyProperty, {x:Bind} and also with re-creating the Run and the Paragraph there instead of directly changing Run.Text from the code - all with the same result.

• The project is targeting Windows 10, version 1803 (10.0; Build 17134)
• Confirmed this bug on 2 boxes running Win 10 Pro 1803 - development and another clean test machine.
• Note that Windows 10 Pro 1809 and a Server build 1809 don't show this bug (the project was still targeting 1803 when tested). There are some quirks how RichTextBlock displays text when user selects a portion of it while the text is modified including temporary disappearance similar to the bug above, but in 1809 a) disappearance is not permanent and requires not just a click, but a selection in progress, b) it's a gray area how the control should behave if user-selected text was modified by the application mid-flight, and c) there are easy and logical workarounds there, e.g. pause automatic text changes if user tries to select something.

Update 2018-12-11

• VS.Net was updated to 15.9.4 that also required projects targeting 1803 downgrade(!) Microsoft.NETCore.UniversalWindowsPlatform version from 6.2.2 down to 6.1.9. The bug is still reproducible in Windows 10 Pro 1803.