The observed behavior isn't completely clear, when painting an absurd amount of Detail Meshes onto a Unity Terrain object it appears the Detail Meshes will pool up into a corrugated pattern at the edge of Terrain Tiles. This causes previously painted Detail Meshes on the other side of the Terrain object to 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. Without changing any painting settings, paint the sphere onto the Terrain.

Actual Results:
Detail Mesh objects that you paint will be placed and then dissappear if it's not placed along the edge/corner of a Terrain Tile.

Expected Results:
User would be able to Paint Detail Meshes onto the Terrain object and previously placed Detail Meshes would not disappear.