Must be something special with the diagram I guess. Have you tried with a new diagram?
I just tried this little (VB)script in v16.1 on a qea model, and it works just fine
sub main
dim diagram as EA.Diagram
dim package as EA.Package
set package = Repository.GetPackageByGuid("{A5CBF53F-2033-4c9a-B505-3341E5595393}")
set diagram = Repository.GetDiagramByGuid("{9585B09C-28C5-485b-AF25-79F1D70EAA18}")
dim diagramObjects as EA.Collection
dim diagramObject as EA.DiagramObject
set diagramObjects = diagram.DiagramObjects
set diagramObject = diagramObjects.AddNew("l=10;r=100;t=10;b=200;", "")
diagramObject.ElementID = package.Element.ElementID
diagramObject.Update
end sub
main
Geert