Reproduction steps:
1. Open attached project "RetinaChange"
2. Open scene "Demo"
3. Enter play mode
--- In the console you should see screen width and height printed
--- They differ in Awake and in Start
--- Could not reproduce in standalone, only in editor

Code that gives strange results:

void Awake() {
Debug.Log("AWAKE: SCREEN SIZE IS " + Screen.width + ", " + Screen.height + "(" + Screen.dpi + ")");
}
void Start () {
Debug.Log("START: SCREEN SIZE IS " + Screen.width + ", " + Screen.height + "(" + Screen.dpi + ")");
}

Regression introduced in 5.4