Reproduction steps:
1. Create a new project
2. Create > UI Toolkit > UI Document
3. Double-click on the newly created Asset to open UI Builder window
4. Drag and drop any of the Elements into the Hierarchy
5. Expand Material foldout in the Inspector
6. Select any of the Materials and observe the Select Material window

Actual result: Materials flicker to default Material icon and to smaller Material icon
Expected result: Nothing of note happens

Reproducible with: 6000.3.7f1, 6000.4.0b7, 6000.5.0a6
Couldn’t reproduce with: 6000.0.66f2 (No Material field)

Reproduced on: Ubuntu 24.04
Not reproduced on: Windows 11, macOS Sequoia 26.2 (M4)