When building a tree using the MVVM template, like:

<tree id="categoryTree" model="@load(vm.categoryModel)" rows="12">
<treecols>
<treecol label="Category title" />
<treecol label="Type" />
<treecol label="Selectable" />
</treecols>
<template name="model" var="c">
<treeitem open="true">
<treerow>
<treecell>
<a label="@load(c.data.title)" onClick="@command('onClickTitle',categoryID=self.data.ID)"/>
</treecell>
<treecell label="@load(c.data.typeTitle)" />
<treecell label="@load(c.data.selectable)" />
</treerow>
</treeitem>
</template>
</tree>

... then setting the Treeitem open attribute to 'true' has no affect. I'm presuming that this is a bug.