How to reproduce:
1. Open attached project ('1150178repro')
2. Click on 'Terrain' object and in the Terrain component, click on 'Paint Terrain' tool
3. Set the tool to 'Set Height'
4. Hold shift (or control, depending on what the tooltip says) and click on any part of the terrain

Expected results: Returned height is only relative to the Terrain object (0 on flat terrain)
Actual results: Returned height is relative to the Terrain object + the object's Y coordinate in world space

Reproducible with: 2018.3.14f1, 2019.1.1f1, 2019.2.0a10, 2019.3.0a1
Not reproducible with: 2017.4.26f1, 2018.3.6f1, 2019.1.0a8
Regressions: 2018.3.6f1 -> 2018.3.7f1 and 2019.1.0a8 -> 2019.1.0a9
Could not test with later 2019.2 versions due to the project immediately crashing.

Note: Since the height of Terrain can only fit between 0 and 600, when the Terrain object is outside these bounds, flat terrain for example will return either 0 (when Y is negative) or 600 (when Y is positive).