In the definition of StackElementType, there is the property NumberInList which comes from the interface IOrderedObjectType. The interface IOrderedObjectType defines that this property is read only. But looking at the definition of StackElementType into the nodeset file https://github.com/OPCFoundation/UA-Nodeset/blob/UA-1.04.10-2021-09-15/IA/Opc.Ua.IA.NodeSet2.xml (line 1344), the property NumberInList is writable, access level is 3 (line 1365):

<UAVariable NodeId="ns=1;i=6015" BrowseName="NumberInList" ParentNodeId="ns=1;i=1005" DataType="UInteger" AccessLevel="3">

The Part 200 does not give any reason why this has been made writable, and in the amendment 13 is stating that two objects in the list shall never have the same value of NumberInList, but if writable, it will be impossible to ensure.

Either there is a good reason for that and the IA spec shall state it explicitly or the model file and resulting nodeset shall be fixed.