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

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.

GridViewDataColumn doesn't respect FallbackValue when the source property returns DependencyProperty.UnsetValue

A kind of a workaround for the user: resize any column/the entire window or collapse the row details.