Here’s what I just tried:
<!DOCTYPE html>
<html>
<head>
<title>Minimal GoJS Sample</title>
<!-- Copyright 1998-2018 by Northwoods Software Corporation. -->
<meta charset="UTF-8">
<script src="go.js"></script>
<script id="code">
function init() {
var $ = go.GraphObject.make;
myDiagram =
$(go.Diagram, "myDiagramDiv",
{
initialContentAlignment: go.Spot.Center, // for v1.*
layout: $(go.TreeLayout, { isInitial: false }),
"ModelChanged": function(e) {
if (e.isTransactionFinished) document.getElementById("mySavedModel").textContent = e.model.toJson();
},
"undoManager.isEnabled": true
});
myDiagram.nodeTemplate =
$(go.Node, "Auto",
{ locationSpot: go.Spot.Center },
new go.Binding("location", "loc", go.Point.parse).makeTwoWay(go.Point.stringify),
$(go.Shape,
{ fill: "white", portId: "", fromLinkable: true, toLinkable: true, cursor: "pointer" },
new go.Binding("fill", "color")),
$(go.TextBlock, { margin: 8 },
new go.Binding("text"))
);
myDiagram.linkTemplate =
$(go.Link,
{ reshapable: true, resegmentable: true },
new go.Binding("points").makeTwoWay(),
$(go.Shape),
$(go.Shape, { toArrow: "OpenTriangle" })
);
load();
}
function load() {
var str = document.getElementById("mySavedModel").value;
myDiagram.model = go.Model.fromJson(str);
}
</script>
</head>
<body onload="init()">
<div id="myDiagramDiv" style="border: solid 1px black; width:100%; height:600px"></div>
<textarea id="mySavedModel" style="width: 100%; height: 300px">
{ "class": "go.TreeModel",
"nodeDataArray": [
{"key":1, "text":"Alpha", "color":"pink", "loc":"25 41.3"},
{"key":2, "text":"Beta", "color":"orange", "loc":"122 15.65", "parent":1, "points":[50,41.3,60,41.3,90,15.65,100,15.65]},
{"key":3, "text":"Gamma", "color":"lightgreen", "loc":"131.5 66.94999999999999", "parent":1, "points":[50,41.3,60,41.3,90,66.95,100,66.95]},
{"key":4, "text":"Delta", "color":"pink", "loc":"227.49999999999986 125.94999999999996", "parent":3, "points":[163,66.94999999999999,173,66.94999999999999,231.3333282470703,23.333328247070312,134.3333282470703,164.3333282470703,194,125.94999999999999,204,125.94999999999999]}
]}
</textarea>
</body>
</html>
This loads as:

Note the { isInitial: false }
on the TreeLayout.