Reproduction steps:
1) Open project "LoadEventBug"
2) Open scene "Demo"
3) Play the scene.
--- Note that "init" is logged to the console.
--- Note that "loaded" is not.
--- This is despite the Init function supposedly being called before the onSceneLoaded event is fired

Code:

public class Test : MonoBehaviour
{
[RuntimeInitializeOnLoadMethod(RuntimeInitializeLoadType.BeforeSceneLoad)]
private static void Init()
{
Debug.Log("init");
SceneManager.sceneLoaded += LevelWasLoaded;
}

private static void LevelWasLoaded(Scene scene, LoadSceneMode mode)
{
Debug.Log("loaded");
}
}