When you create a new Terrain asset within the Unity Hierarchy or from the Terrain Tools Toolbox, a new Terrain.asset file is created within your Assets folder. When you attempt to Undo the creation of this new Terrain Gameobject in your scene. The Gameobject is removed but the .asset file remains.

Repro Steps:
1. Create a Terrain Object in your scene
2. Observe the Terrain.asset file that was created in your Assets folder
3. Press Ctrl-Z to Undo the creation of the Terrain Object

Actual Results
Terrain Asset file remains in the Assets folder but the Terrain Gameobject was removed from your scene.

Expected Results:
Undo action would undo all actions that occurred during the process of creating a Terrain Gameobject in your scene