When a user Splits Terrain by using the functionality from Terrain Tools, any painted on Detail Meshes will disappear.

Repro Steps:
1. Launch Unity Editor Version: 2020.1.0b9.3653
2. Create a Terrain
3. Create a Prefab of a Sphere
4. Select the Terrain then go to Paint Details
5. Add the Sphere as a detail mesh
6. Paint Spheres onto the Terrain
7. Open up the Terrain Tools Window, go to Terrain Utilities -> Split (Ensure you have Keep Original Terrain) unchecked

Actual Results:
When the Terrain is split the painted Detail Mesh data disappears

Expected Results:
The Detail Mesh data would be retained once the Terrain is split