When you select a terrain and try to paint/raise/lower it, the currently selected terrain should get that modification.
As it is right now, clicking to paint also selects the terrain again, sometimes switching the modification to a newly selected one.

Repro:
1. Download this project or start a new project with 2 overlapping terrains
2. Select Terrain(1)
3. Chose to raise it with a random brush
4. When you try to paint the modification, notice that after one click, the selection switches to the first Terrain object.

Reproduced in: Unity 2018.2.0a10, 2018.3.0b12, 2019.1.0a11

Not reproduced in Unity: 2018.3.0a9