Instead of isolating which closure is holding the strong reference in this situation, I believe a much safer solution would be to make the reference to the view controller weak in every closure's capture list. Why would a closure need to retain a reference to a view controller after the view controller (and the view it manages) has disappeared?

If you are using the view controller as a "conduit" to pass to a closure a reference to a property that survives the disappearance of the view controller, hopefully you can refactor your code to pass to the closure a reference to that property independent of the view controller.