I have a page view controller that transitions between view controllers using the normal UIPageViewController methods. Inside one of the presented view controllers (let's say the current view controller), I add a child view controller into a subview called
let storyboard = UIStoryboard(name: "Main", bundle: nil)
let child = storyboard.instantiateViewController(withIdentifier: "child")
child.view.frame = self.containerView.bounds
Seems as though something was going on with the storyboard view controllers, as I was able to remove the slider and add a new slider in and the functionality worked correctly inside the UIPageViewController page.