To further expand on Gui’s reply, Dynamo can only listen to Revit events while the graph is executing and dynamo can only execute inside of Revit’s idle event.
Revit’s idle event is only triggered when no other interaction is happening and when it’s activated, Revit’s context is busy until whatever triggered the idle event has completed it course.
Therefore, even if you subscribe to an event during the graph’s execution, that event won’t be triggered by anything that is the result of the same graph.