Steps to repro:

1. Open attached project;
2. Go to Project folder;
3. Multiselect 4 .spm tree prefabs;
4. Observe preview rendering in Inspector.

Expected result:
4 different trees are rendered corrently in correspondance to their tree mesh (see attached expected.png screenshot).

Actual result:
- Only 2 of 4 trees are rendered in preview window in 2017.3;
- All 4 trees are rendered overlayed and mixed in each of 4 preview windows in 2017.2.
See attached actual.png screenshots.

Regression introduced in Unity 2017.2.0.

Notes:
- Reproducible in 2017.3.0a3, 2017.2.0b7;
- Not reproducible in 5.6.1p2;
- Reproducible on Windows 10 and OSX 10.12.5.