This is as expected. Sprite UVs are calculated the first time it is rendered.

A.sprite = Sprite.Create(texture2D, new Rect(0, 0, 64, 64), Vector2.one * 0.5f);
yield return null; <- Since rendering is happening here, the UV for the Sprite is now set to take up the whole sprite
yield return null;

B.sprite = Sprite.Create(texture2D, new Rect(0, 0, 64, 64), Vector2.one * 0.5f);
texture2D.LoadImage(File.ReadAllBytes(Application.dataPath + @"\A_Simple_128x128_Image.png"));
C.sprite = Sprite.Create(texture2D, new Rect(0, 0, 128, 128), Vector2.one * 0.5f);
// When the first render happens, B.sprite will be calculated based on the the new data in texture2D which is now 128x128