RichEditBox math mode content of matrix disappears

[gaviny82]

Describe the bug

In the math mode of RichEditBox, the last row of a matrix with 3 rows disappears if content is added to the first row.

Equations: ⁅■8(&&@&&@&&)⁆, ⁅■8(1&2&3@&&@&&)⁆, ⁅■8(1&2&3&4@&&&@&&&)⁆

The matrix can display correctly if the first row is left empty, or something larger is entered:

Equations: ⁅■8(&&@4&5&6@1&2&3)⁆, ⁅■8(1&2&3@&&@&&)+■8(1@2@3@4)⁆, ⁅■8(1&2&3@&&@&&)+∑_𝑖⁆

This issue only affects matrices of 3 rows. Matrices of other sizes can be displayed correctly:

Equation: ⁅■8(1&2&3&4@&&&@&&&@5&6&7&8)⁆, ⁅■8(1&2&3@&&@&&@4&5&6)⁆

Steps to reproduce the bug

1. Create a WinUI 3 App project with Window App SDK 1.7-exp1.
2. Add a RichEditBox with a reasonable size to the page.

<RichEditBox x:Name="editor" Width="800" Height="500 />

3. Set the RichEditBox to math mode.

public sealed partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
        editor.Document.SetMathMode(RichEditMathMode.MathOnly);
    }
}

4. Create a matrix with 3 rows and add elements to the first row. Test the sample equations in the description.

Expected behavior

Screenshots from OneNote:

Screenshots

No response

NuGet package version

WinUI 3 - Windows App SDK 1.7 Preview 1: 1.7.250208002-preview1

Windows version

Windows 11 (24H2): Build 26100

Additional context

No response