The reason for declining the item is that a possible fix for this behavior would practically interfere with the virtualization mechanism of the control. Shortly said, when the columns have their width initially set, RadGridView is aware of the column with the biggest size and performs its calculations based on it. When their size is changed on loading, the control cannot be aware of the size of a column that is not present in the viewport thus, the reported issue is observed. In order this behavior to be modified so, the engine needs to measure an element that is not yet loaded. We cannot commit ourselves to implementing this, as this would affect the virtualization of RadGridView. A possible workaround would be to avoid setting the Width of the columns initially in XAML.

Available in LIB version 2017.3.1127, it will be also available in the R1 2018 Release.

This is reproducible in a hierarchical scenario. The nested gridview loses its selection when its parent row is scrolled out of the viewport.

Edit:
We are declining this bug as originally the export options were not meant to respect group properties but only some global GridView properties.
However we will add such option in GridViewDocumentExportOptions class.
You can follow this feature request => 
https://feedback.telerik.com/Project/143/Feedback/Details/229211-gridview-add-option-in-gridviewdocumentexportoptions-to-include-exclude-grouph

Currently, the following workaround can be applied:

private void RadGridView_Loaded(object sender, RoutedEventArgs e)
{
	var gridView = sender as RadGridView;
	foreach (var column in gridView.Columns)
	{
		if (!column.IsResizable)
		{
			Dispatcher.BeginInvoke((Action) (() =>
			{
				var radGridView = column.Parent;
				foreach (var header in radGridView.ChildrenOfType<GridViewHeaderCell>())
				{
					if (header.Column.DisplayIndex == column.DisplayIndex)
					{
						var leftGripper = header.ChildrenOfType<Thumb>().Where(x => x.Name == "PART_LeftHeaderGripper").First();
						{
							leftGripper.Visibility = Visibility.Collapsed;
						}
					}
					else if (header.Column.DisplayIndex == (column.DisplayIndex - 1))
					{
						var rightGripper = header.ChildrenOfType<Thumb>().Where(x => x.Name == "PART_RightHeaderGripper").First();
						{
							rightGripper.Visibility = Visibility.Collapsed;
						}
					}
				}
			}), DispatcherPriority.Render);
		}
	}
}

Add option in GridViewDocumentExportOptions to include / exclude GroupHeaderRowAggregates.

This will allow users to hide/show group header row aggregates in the exported documents.

Option will be globbally applied - to all group header rows in the RadGridView.
===
From 2017 R3 SP GridViewDocumentExportOptions  adds ShowGroupHeaderRowAggregates bool property.

It can be used like so:

  GridViewDocumentExportOptions options = new GridViewDocumentExportOptions()
            {
                ShowGroupHeaderRowAggregates = true
            };
this.gridView.ExportToWorkbook(options);

Currently, you can use only the predefined one - IsEqual, IsNotEqual, etc. Add an option to create operators that execute custom filtering action. Also, add support for modifying the default operators.

Currently, if the value set to the RowHeight of RadGridView is lower than the Height of the highest cell(or the Height of a Hierarchy defined), it will not be respected. Provide an option the RowHeight to have greater weight than the height of the cell.

When we make frozen column width bigger than the window will hide other columns. Scrolling is applied only to not frozen columns. So we can't scroll to see other columns.

A possible workaround is to set MaxWidth property of the column.

I am setting the status of the item to Declined as the reported behavior is due to the fact that the RowDetailsVisibilityMode is set to VisibleWhenSelected. The default behavior of RadGridView is to split the merged cell when a row with expanded row details is added. Thus, the reported issue you are is the expected one. If setting the RowDetailsVisibilityMode is removed the control will behave as expected. 

Available in LIB version: 2017.2.703

Currently VQCV caches all the items once they are loaded. In this way the data virtaulization functions only through the first bottom-most item scrolling. The idea behind this item is to make this behavior optional by introducing an alternative mode.

If you set BorderThickness through a custom CellStyle, the left border is overwritten by our code.