Searching in grouped ListView in Xamarin.Forms

Background

I followed this guide to create a grouped ListView in Xamarin.Forms. Doing so, the following Property is given which is bound as the ItemSource of the ListView:

DevicesGrouped = new ObservableCollection<Grouping<string, Device>>(sorted);

Everything works just fine, including the grouping functionality.

To search (and thereby filter) the entries in the list, I have tried to implement this pattern:

this.ItemsSource = DevicesGrouped
        .Where (x => x.Name.ToLower ()
        .Contains (filter.ToLower ()));

The issue

The problem is, that I do not have access to the properties of the Device as the objects reside in Groupings. I can only access the Key of the DevicesGrouped which, in this case, is a Manufacturer, where I want to search for the Name. The problem in searching for the Key is also, that the ListView scrolls to the position of the Key and thus hides the elements in the ListView behind the grouping header.

My question is thus, how would one access the properties of the ObservableCollection which is Grouped?

I have tried keeping a List which I use when I filter, but doing so, the app crashes, since Groupings are enabled on the ListView itself.

Thanks for the help in advance.