I have a custom RouterEventNode where I override OnReset() to set the target to null (Because the parent behavior tree is re-used with a different agent).
I figured out that OnReset() is never called because the Node has a Resting state at the time the graph is stopped and therefore the Reset method returns early. So, there seem to be no way to reset a node inside a sub-graph (not sure if sub-graph is actually the problem here).
In regards to Flowscript nodes (FlowCanvas), you can use/override the OnGraphStarted or OnGraphStopped if you want to set/reset some values for the node. (also please make sure to call base method in case you override any such method).
Please let me know if using either of the two methods work for you.