Brackets/parentheses are invisible in RichEditBox MathMode

[BreeceW]

Describe the bug

Math characters that group other expressions become invisible when typed into WinUI 3 RichEditBox with math mode. This is affecting the initial availability of math mode in Windows App SDK 1.7-experimental2. This is a regression from the system XAML (WinUI 2) math mode.

Some examples of the expressions affected:

• Parentheses/brackets ( ) { } [ ]
• Summation ∑
• Integrals ∫
• Likely many more

Steps to reproduce the bug

1. Add a RichEditBox and set RichEditBox.TextDocument.SetMathMode(RichEditMathMode.MathOnly)
2. Type a mathematical expression with parentheses, such as f(x)=x
3. Notice that the parentheses become invisible once the equals key (in that example) is pressed

Also reproducible with expressions like summation:

1. Type \sum^n (and hit space to commit)
2. The summation ∑ becomes invisible

Expected behavior

All grouping elements should remain visible. See below screen recordings for expected behavior in UWP/WinUI 2.

Screenshots

WinUI 3 behavior	Expected behavior (WinUI 2)

NuGet package version

Microsoft.WindowsAppSDK 1.7.250109001-experimental2

Windows version

Windows 11 (24H2): Build 26100

Additional context

The problem seems to be just rendering. It's still possible to type in the invisible areas, and text copied out is correct.