STR:
1. Open "main" scene in the project
2. Play the scene
3. Observe the additive scene is not loading and object not instantiating
4. Switch scene and resource load lines with each other:
From:
m_ao = SceneManager.LoadSceneAsync("Stage", LoadSceneMode.Additive);
m_rr = Resources.LoadAsync<GameObject>("Model");
To:
m_rr = Resources.LoadAsync<GameObject>("Model");
m_ao = SceneManager.LoadSceneAsync("Stage", LoadSceneMode.Additive);
5. Observe the scene loading and object instantiating

Reproduced on: 5.3.6f1, 5.3.6p8, 5.4.1p1, 5.4.2f2, 5.5.0b9