According to the provided screenshot, it seems that you are using the Fluent theme to your application. I have tested the provided code and it displays the message box as expected:
Have you changed the font of the message box? Because the different fonts may result in different result when rendering the text.
By default we use GDI+ to measure the text, however, GDI+ is a known suspect for reporting wrong sizes of the text and this is also the case here. You can easily force the element to use GDI instead of GDI+ by setting the UseCompatibleTextRendering property to false and this should improve the rendering of normal text. However, for HTML-like text rendering, the possible solution that I can suggest is to use a suitable font.
I am looking forwrad to your reply.
Dess | Tech Support Engineer, Sr.
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