Dark rendering artifact appears in Graph Toolkit space when placing Placemat or editing Placemat color in Graph Inspector

Steps to reproduce:
1. Create new Unity project
2. Import the "MySimpleGraph" script
3. In the Project window, right-click -> Create -> Graph Toolkit Samples -> Simple Graph -> Open the Graph
4. In the Canvas, right-click -> Create Placemat
5. In Graph Inspector, select Color field
6. Change Placemat color
7. Observe the Graph Canvas

Actual results: A dark appears in Graph Canvas after creating Placemat or editing Placemat color

Expected results: No rendering artifacts should be visible in Canvas after creating Placemat or editing Placemat color

Reproducible with versions: 6000.4.0b7, 6000.5.0a6

Tested on (OS): Ubuntu 24.04 (reproducing), macOS Silicon Tahoe (M1) (not reproducing)