Steps to reproduce:
1. Open project
2. Open 'Main' scene
3. Build and run this project and keep tracking console

Results: Console prints that SceneManager.GetActiveScene().buildIndex: -1 when Player shows that index is 0.

Note: Scene.buildIndex never returns a valid index unless in play mode. This appears to include all ways of obtaining Scenes from either SceneManger or EditorSceneManger while in Edit mode or during the build process.

Reproducible with: 5.4.0b20; 5.3.5f1;
Not reproducible with: 5.2.4f1