It seems that problem sometimes appears when item is moved from RadListBox1 - SourceListBox to  RadListBox2 - DestinationListBox.

Anyway, if item is moved from one listbox to the other one by one with the arrow, it works good. Item is assigned the biggest SortOrder number and appears last in sorting order.

However, move of multiple items (or all items) at the same time, creates the same SortOrder value for ALL the moved items. Is that expected behavior? I removed move all button since it makes it very hard to sort items once they all have the same SortOrder value.