Holes and different colors appear on default Tree Materials after applying them to the Tree GameObject and undoing changes

Reproduction steps:
1. Create a new 3D (Built-in Render Pipeline) template project
2. Create a Tree GameObject (GameObject > 3D Object > Tree)
3. Select newly created GameObject and press “Add Leaf Group” button in the Inspector
4. Assign Bark Material to the Branch and Leaf Material to the Leaf and Apply changes
5. Add Tree Renderer to the LOD Group and observe the Preview window
6. Undo changes several times and observe the Tree GameObject in the Scene view and Material icon in the Project window

Actual result: Holes appear on the Tree and Material icons
Expected result: Changes are reverted

Reproducible with: 6000.0.65f1, 6000.3.4f1, 6000.4.0b4
Couldn’t reproduce with: 6000.5.0a4 (No 3D (Built-in Render Pipeline) template)

Reproduced on: macOS Tahoe 26.2 (M4), Ubuntu 24.04

Note: Several different colours appear on the Tree and Material icons if Material colors are changed to non-default ones