Reproduction steps:
1. Open the attached project "ReproProj"
2. Open the “/Assets/Scenes/FirstScene.unity” Scene
3. Enter the Play Mode
4. Observe the Console

Expected result: No Errors exist, and the text is rendered correctly on hover
Actual result: Errors “NullReferenceException: Object reference not set to an instance of an object“, “UnityException: HasProperty can only be called from the main thread.“, “ArgumentException: The state of the provided MeshGenerationNode is invalid (entry is null).“ are present, and the text is rendered broken on hover

Reproducible with: 6000.0.60f1, 6000.2.10f1, 6000.4.0a3
Not reproducible with: 6000.3.0b7 (Assets\Plugins\UniRx\Scripts\UnityEngineBridge\Triggers\ObservableJointTrigger.cs(24,29): error CS1069: The type name 'Joint2D' could not be found in the namespace 'UnityEngine')

Reproducible on:
Play Mode

Could not test on:
Windows Standalone Player (Build fails)

Testing environment: Windows 10 Enterprise 21H2
Not reproducible on: No other environment tested

Notes:

In contrast, everything works fine when using the TextSettings file 'UltraStarPlayTextSettingsFixed' instead of 'UltraStarPlayTextSettings'.
The only difference is that the option 'Fallback Material Settings > Match Material Settings' seems to cause the bug
The project uses custom TextSettings with custom Fallback Fonts
MaterialIcons and FontIcons is a custom UXML that render Google Material Icons from a font asset
Some common code is moved to separate Packages named "playshared" and playsharedui". The fonts, FontIcons, styles, etc., are in “Packages/playsharedui”. You also find it when inspecting the UIDocument > PanelSettings > TextSettings