Reproduction steps:

1. Open "MinimalRepro" project
2. Open "MapEditor" scene
3. Enter Play Mode
4. Select "Precipitation Particle System" in the Hierarchy window and notice its properties (e.g. "Start Size")
5. "New Map" -> "Checkmark"
6. "Map Settings" -> "Conditions" (Flag)
7. in "e.g. Weather,Snow" write "Weather, Snow"
8. Look at "Precipitation Particle System" properties (e.g. "Start Size")

Expected Result: The Particle system properties should change according to the script
Actual Result: The Particle system properties don't change

Reproduced with: 2018.3.0a8, 2018.2.4f1, 2018.1.9f1, 2017.4.9f1, 2017.3.2f1, 2017.2.3p3, 2017.1.5f1

Note:
Only changes the material