UINavigationController weird transparency behaviour when presenting - swift

Question

I'm experiencing a weird glitch with the transparent navigation controller. When presenting a view controller with a UIImage at the top of the navigation controller: at first presents half of its background blur with a dark section, and half with a clear one; and after a very short moment it changes for a full dark background. As shown in the gif:
The UIImage displayed is mostly white, the borders are very close to pure white. So the grey color does not make much sense (less sense makes the rapid change).
I did disable extend edges under top bar in the Storyboard for the presented view controller. So the image is not hidden behind the Navigation Controller. With this option enabled the glitch does not appear, but I don't want to hide part of the image.
Disabling the transparency, solves my problem, but I'd like to be able to keep the transparency effect.
Edit: I did notice the "grey effect" also makes the navigation controller opaque. But only for that view, when going back, is transparent again.
Xcode 11, Swift 5, iOS 13.2, iPhone XS.
Thank you very much for your help.

Answer 1

I finally found the problem.
I didn't want the UIScrollView to bounce on top so I used this code:
extension ProblematicViewController: UIScrollViewDelegate {
/// Prevent bounce at top
func scrollViewDidScroll(_ scrollView: UIScrollView) {
if scrollView.contentOffset.y < 0 {
scrollView.contentOffset.y = 0
}
}
}
Which meant the ViewController's Adjust Scroll View Insets were overwrite (I think). So my scroll view did lay under the UINavigationController, to prevent this from happening, I did disable Extend Edges: Under Top Bars, so the root UIView containing the UIScrollView was not covered by the UINavigationController. So the transparency effect could never happen because the view was never under the top bar. More about Adjust Scroll View Insets on this post (helped me to find the problem).
Removing those lines and enabling Adjust Scroll View Insets and Extend Edges: Under Top Bars, solves the glitch problem, but the UIViewController does not have the "prevent bounce on top" behaviour that I wanted.
This kinda explains the weird behaviour of the UINavigationController because the SO has to calculate a transparency over nothing, but is sill strange, the transparency effect is calculated when the ProblematicViewController is fully presented, and not on viewDidLoad() or another view's life cycle before is shown to the user, so the transition is smooth, even when nothing lays behind the effect.