Reproduction steps:
1. Open the attached project named Case_1186005
2. Open the Sample Scene and select the Terrain
3. Hold Shift and try to smooth the Terrain near the edge of the Tile
4. Observe the Terrain

Expected result: The Terrain is smoothed out without any issues
Actual result: Terrain becomes terraced

Reproducible with: 2019.1.0a1, 2019.2.8f1, 2019.3.0b5, 2020.1.0a7
Could not test with: 2017.4.32f1, 2018.4.10f1(Terrain Tools were not yet available)

Package versions tested: 1.1.2, 2.0.0, 2.0.2

Notes:
1. Terrain Tools became available in 2019.1.0a1