The SelectedCells collection of RadGridView contains wrong cells in a scenario where the previous selection is cleared and then its items are filtered out of the view. This reproduces with SelectionMode set to Extended and SelectionUnit set to Mixed.
This happens if you call gridView.SelectedItems.Clear() or gridView.UnselectAll(). You can work around the issue if you clear the SelectedCells. To do so, call also gridView.SelectedCells.Clear().gridView.SelectedCells.Clear();