flows in StructuredActivityNode are not drawn

[vhemery]

In an activity diagram, draw a StructuredActivityNode.

Then create 2 nodes in it (e.g. InitialNode and OpaqueAction).
-> The nodes are contained by the StructuredActivityNode, which is good.

Try and create a ControlFlow between these two nodes.
-> A ControlFlow is created in the model in the StructuredActivityNode, which is good.
-> The ControlFlow is not shown in the diagram, which is bad.

The ControlFlow can be show only when we move the corresponding model element to the parent activity. So my first guess would be that there is a candidates expression which does not look into StructuredActivityNodes.