UPDATE: The issue is reproduced with columns bound to other data types as well.

Respect IVirtualizedItemProvider for items of RadGridView . 

The problem should be resolved with LIB version 2016.1.404.

When tabbing in grouped RadGridView placed in Grid with another visual element inside it, GroupRenderMode is set to Flat  and second group is not in ViewPort, selected row changed its state to Unfocused.

Currently, within the LoadItems() method the items are being iterated directly, thus a second query to the database is triggered.  A separate list for this operation should be created.

Workaround: reused the cell element like this:
CreateCellElement(Telerik.Windows.Controls.GridView.GridViewCell cell, object dataItem)
{
    var button = cell.Content as Button;
    . . .
 
    return button;
}