To reproduce:

Run the attached application. Drag Node 2 to after Node 4 while holding down the Alt key. Note that the Node 2 copy is inserted before Node 4.

When Node 2 is dragged to after Node 4 without holding down the Alt key, Node 2 is correctly moved to after Node 4.

Workaround: you can modify the TreeViewDragDropService and control at what position exactly to be inserted the dragged node:

https://docs.telerik.com/devtools/winforms/treeview/drag-and-drop/modify-the-dragdropservice-behavior
https://docs.telerik.com/devtools/winforms/treeview/drag-and-drop/drag-and-drop-in-bound-mode