ios - Swift - UIProgressView becomes more opaque as time goes on -

so have uiprogressview animates 3 seconds seems opacity of bar @ start, lower @ end. bar seems become more opaque loads. have attached screenshots show mean can't work out why is. characteristic of loading bar or xcode bug? don't think there's wrong code it's pretty simple really:

uiview.animatewithduration(3, animations: { () -> void in             self.progressview.setprogress(1.0, animated: true)         }) 

anyway here screenshots better demonstrate issue: