Default UI Material preview is empty when viewed in the Inspector for a TMP Button

Steps to reproduce:

1. Create a new project using Universal 3D template
2. In the Hierarchy, right-click > UI (Canvas) > Button TextMeshPro
3. Import TMP Essentials if needed and select the created Button
4. In the Inspector, in the preview, click on the "Button" dropdown and switch to "Default UI Material"
5. Observe the preview

Actual results: Preview is empty, fully gray

Expected results: The preview shows a white square (the standard default UI material rendering)

Reproducible with versions: 6000.4.1f1, 6000.5.0b1, 6000.6.0a1

Not reproducible with versions: 6000.3.13f1, 6000.4.0f1 (107f2798865e)

Tested on (OS): macOS 26.3

Notes:

• Converting the Inspector to a Floating Window and then closing it forces the preview to show temporarily. However, the preview breaks again and is empty after reselecting