navigationBar.topItem?.prompt diables buttons (bug?)

If I show a prompt on a NavigationController

self.navigationController?.navigationBar.topItem?.prompt = "something to prompt"

The self.navigationController?.navigationBar.topItem ... barButtonItems, stop working. They don't even indicate with teh standard animation that they are being touched. Is this epected behaviour or bug

navigationBar.topItem?.prompt diables buttons (bug?)
 
 
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