How to reproduce:

1. Open the project “UTK-fontasset-fallback-bug.zip“
2. Open the “FallbackFontBug“ Scene
3. Enter Play Mode
4. Press “Toggle Bold“ button in the Game view
5. Observe the Console and the Game view

Expected result: Nothing is logged in the Console after step 4, text is rendered as before step 4
Actual result: Multiple warnings and errors are being spammed on each click, and the rendered text is broken

Reproducible in: 6000.0.54f1, 6000.0.61f1, 6000.2.10f1, 6000.3.0b8, 6000.4.0a3
Not reproducible in: 6000.0.53f1

Reproduced on: Windows 11 Pro (24H2), macOS 26.0.1 (By the reporter)
Not reproduced on: No other environment tested

Notes:

• The most prominent errors and warnings are:

** “ArgumentException: The state of the provided MeshGenerationNode is invalid (entry isn't empty).“
** “NullReferenceException: Object reference not set to an instance of an object”
* “MeshGenerationContext is assigned to a VisualElement after calling 'UnityEngine.UIElements.UIR.MeshGenerationCallback”
The Text in the Game view breaks