How to reproduce:
1. Open the attached "PostProcessScene" project and "Scene_1" scene
2. Observe the Loader.cs and SceneProcessor.cs scripts. Loader script loads the five scenes in the project additively, SceneProcessor has a method with PostProcessScene attribute that logs the scene count of loaded scene
3. Enter playmode, observe the console output

Expected result: A method with PostProcessScene attribute gets called every time the scene is loaded via SceneManager.LoadScene (or any other SceneManager function)
Actual result: A method with PostProcessScene attribute does not get called after the scene is loaded via SceneManager.LoadScene (or any other SceneManager function)

Note: Issue is reproducible when loading a scene additively or in a single mode, so the used scene loading mode does not seem to matter here

Regression introduced in - 2017.2.0b2
Reproducible with - 2017.2.0b3, 2017.2.0b2
Not reproducible with - 2017.2.0b1, 2017.1.0f3, 5.6.2p4

Fixed in - 2017.2.5f1