This looks like a bug actually; when I make a selection and then try to refine using the refine edge tool, I cannot add OR subtract from the mask (and the mask edge gets really wonky when you first paint or erase).
Can anyone from Adobe confirm? Using PS Touch ver. 1.2 for Android.
I have just used the tool today and I found that the Refine edge tool works best for detailed selections such as hair and fur. I'm not sure whether there is a bug or not, however. I would use the brush selection tool instead to refine the rough selection from the scribble selection tool. Hope this helps!