Only one Terrain Layer is copied to the new Terrain when using the "Create Neighbor Terrains" tool

Steps to reproduce:

1. Create a new Unity project using the Universal 3D template
2. Create a new Terrain in the Hierarchy (Right-click > 3D Object > Terrain)
3. Create 3 different Terrain Layers in the Project window (Right-click > Create > Terrain > Terrain Layer)
4. Select the created Terrain in the Hierarchy
5. Click the "Paint Terrain" button in the Inspector
6. Click the "Edit Terrain Layers" button and add the 3 created Terrain Layers
7. Click the "Create Neighbor Terrains" button
8. Create a Neighbor Terrain by clicking on the side of the Terrain in the Scene View
9. Select the newly created Neighbor Terrain in the Hierarchy
10. Repeat step 5
11. Observe the Terrain Layers

Actual results: Only one Terrain Layer is copied to the new Terrain

Expected results: All layers are copied to the new Terrain

Reproducible with versions: 6000.0.0f1, 6000.0.83f1, 6000.3.24f1, 6000.5.11f1, 6000.6.1f1, 6000.7.0a6

Tested on (OS): MacOS Silicon Tahoe 26.6.2