To reproduce:

1. Open attached project
2. Add a new script to the RawImage Object and add this line to Update():
if(gameObject.GetComponent<RawImage>()!= null) Debug.Log ("Height: " + gameObject.GetComponent<RawImage>().uvRect.height + " Width: " + gameObject.GetComponent<RawImage>().uvRect.width);
3. Build to iOS (or android)
4. Notice that even though uvRect.height and uvRect width are reported to be -1 and 1, they clearly aren't, as evidenced by the RawImage not displaying the full image.