How to reproduce:
1. Create a new Unity project
2. Create a UITK Text Settings Asset
3. In the Inspector window enable the Match Material Preset under Font Assets Fallback → Fallback Material Settings
4. Create and open a new UI Document (UXML), Create → UI Toolkit → UI Document (UXML)
5. In the UI Builder Hierarchy window, add a Label object
6. Change the Labels Text to “<u>Hello World 😁</u>😁”
7. Save the changes
8. Observe the UI Builder window and the Console window

Actual result: The UI Builder window stops displaying text in its Inspector window and the Label object’s text in the Viewport, and errors are thrown in the Console window
Expected result: The “<u>Hello World 😁</u>😁” is correctly displayed and no errors are thrown

Reproducible with: 6000.0.71f1, 6000.4.0a2 (79bd4f638ac0), 6000.5.0a4
Not reproducible on: 6000.3.12f1, 6000.4.0a1, 6000.4.1f1
Could not test with: 6000.5.0b1, 6000.6.0a1 (Fallback Material Settings are not available in these versions)
Fixed in: 6000.5.0a5

Reproducible on: macOS 26.3.1 (by user), Windows 11
Not reproducible on: No other environments tested

Notes:
- The UI Builder window can also become nonresponsive, and after closing it, you are not able to open it again or open another UI Document
- Errors thrown:
-- IndexOutOfRangeException: Index was outside the bounds of the array
-- NullReferenceException: Object reference not set to an instance of an object