After stating exactly the scenarios and noting the two differences...
I removed the Navigation controller from one of the cases, and it still doesn't work.
It seems that the "problem" is caused by doing the "to" segue from the Detail page of the SplitView.
Here's some info:
In iOS13, starting the Unwind segue causes a call to "allowedChildrenForUnwinding(from:)" when it is present in the top SplitView Controller. I call the Super of that in my override and the children array contains the ViewController of the Detail page (which is where I really want it to go).
In iOS 12, starting the Unwind segue causes a call to that same routine BUT in the ViewController I'm trying to dismiss! The Super call returns an array with 0 elements. I tried returning the view controller I'm trying to get to and that doesn't make any difference. (The "allowedChildren..." in the top SplitView Controller is never called.
It is difficult to say without code.
But may be you have a problem with the navigation.
If you have a navigation, you should have a back button. So why need to exit ?
ANother possible cause is with fullScreen vs Automatic presentation.
Could you change, in Attributes inspector, for bot navigationController and its child, presentation to FullScreen instead of automatic and see if you get a problem in iOS 13.
Thanks for your reply Claude. I was using fullScreen already.
I finally figured it out. The issue was that, in iOS12, the Unwinds are unwilling to return to the top SplitView Controller when the unwind-from VC was reached from the Primary page of the SplitView. All I had to do was create an unwind-action function in the VC for the Primary page (similar to the one I already had in the top SplitView Controller) and select that new action when I control+dragged the Done button to Exit.
This works in both iOS13 and iOS12.
(Yes, I probably shouldn't have navigation controllers because I'm not pushing and popping, but as it's turned out, now that I am unwinding to the VC that presented the page, I could be using the navigation controllers functions.)