LayerConstraints are not enforced when Subgraph is present

I think I've found an issue with Graph.LayerConstraints.AddUpDownConstraint / Graph.LayerConstraints.AddLeftRightConstraint.
In a simple graph, the feature works fine but, when using subgraphs, constraints are no longer enforced.

I figured out the Dictionary<Node, int> nodeIdToIndex fields in VerticalConstraintsForSugiyama / HorizontalConstraintsForSugiyama might be related to this issue.
When using subgraphs, some Layout.Node duplication occur and the Node keys in the dictionary never match.
As a consequence, int NodeIndex(Node node) always returns -1.

For example, the following graph has no layout constraints:

If I add a layout constraint where all nodes must be ordered from top to bottom in lexical order, I should get the following graph:

The latest image was generated using a hack where I changed Dictionary<Node, int> nodeIdToIndex into Dictionary<string, int> nodeIdToIndex and using node.ToString() as keys.
Without this hack, I get the same graph as the one with no constraint.

Start by inspecting VerticalConstraintsForSugiyama and HorizontalConstraintsForSugiyama, especially their nodeIdToIndex dictionaries and NodeIndex(Node node). Reproduce the reported top-to-bottom lexical constraint on a graph containing subgraphs, then verify that constraints are enforced without changing the keys to strings or relying on the linked hack.