How to reproduce:
1. Open attached project ("TerrainTreeBrushOptions.zip")
2. Open a Scene called "Scene1"
3. In Hierarchy select Terrain
4. In Inspector click on Paint Trees button
5. In Trees tab select "TreePrefab" tree
6. In Settings tab change "Brush Size" from 40 to 1
7. Enter Play Mode
8. Exit Play Mode
9. Select Terrain -> Paint Trees -> "TreePrefab"

Expected result: All changed Paint Trees "Settings" are saved. In our case "Brush Size" is 1
Actual result: All "Settings" are reverted to default values

Reproducible with: 2018.2.0a5, 2018.1.0b12, 2017.4.0f1, 2017.3.2f1, 2017.2.2p1, 2017.1.3p2, 5.6.5p2, 5.6.0a1

Note: Every other Terrain tool (including Paint Details), after entering Play Mode, saves user selected settings