To reproduce:
1. Open any project
2. Create one terrain
3. Create a Neighbor Terrain
4. In Terrain's Inspector window, select Paint Trees
5. Try painting trees on the touching edges of the two terrains

Actual: trees are only painted on one terrain (disregarding the brush highlight)
Expected: Trees are painted in the area highlighted by the brush (even when it spans across terrain tiles), just like when painting the heightmap.

Reproduced in: 2017.4.20f1, 2018.3.6f1, 2019.1.0b4, 2019.2.0a5