Hi Joao,
Yes, it only occurs on IOS and it's fancy scroll :D
It seems that it's a regular problem, as you can see here or here.
Since I don't know how to change the webview background-color I would go and set the higher class with background-color :#fff;
Perhaps afecting the background color of the classes 'screen' or 'screen-container' does the trick.
Hope it works