How to reproduce:
1. Create a new Unity Project
2. Create a Terrain GameObject
3. Using the “Raise or Lower Terrain” tool, draw some terrain
4. Navigate to Edit > Undo Create terrain
5. Hover the Terrain painting tool over the Terrain
6. Observe Scene view

Actual result: The Terrain painting tool follows the Terrain curvature that existed before the undo action
Expected result: The Terrain painting tool follows the current Terrain curvature

Reproducible with: 6000.0.67f1, 6000.3.7f1, 6000.4.0b7, 6000.5.0a4, 6000.5.0a6
Not reproducible with: 6000.5.0a3

Reproducible on: Windows 11, macOS (15.7.3) (by user)
Not reproducible on: No other environments tested