TreeView Nodes Always Have Checked = True On Postback Even When Not Checked In UI

I have a treeview in my .aspx: <asp:TreeView ID="tvDocCatAndType" runat="server" /> Not much else going on in the page -- two <asp:LinkButtons> and one <asp:Label>; the page is a child of a master page, so these controls are within a <asp:Content> control. I populate the treeview in code -- just 3 node levels, including the root node. All nodes have checkboxes, and I initialize all node.Checked to true. I have some Javascript to do the usual check/uncheck up and down the tree as parent and child node checkboxes are toggled.

No matter how many checkboxes I clear in the UI, on postback every single node has node.Checked = true regardless of the state of the checkbox in the UI. This is not the first time I've used a treeview, but I've never had this problem before. I created this page by light adaptation of an earlier project that works fine.