The following is verbatim of a feedback report (FB22367951) I submitted, shared here as someone else might be interested to see it.
I have reproduced this bug on iPadOS 26.3.1 (a) and 26.4.
During scene resizing on iPad, UIPageViewController's setViewControllers method fails to do its work. The navigation starts and for a brief moment you can see the new view controller coming from the expected direction, but shortly after it fails and stays on the same [current] view controller. It doesn't even call the completion handler when it fails.
When the navigation succeeds (due to not resizing a scene during the navigation) after previously failing at least once, the completion handler is sometimes called more than once.
I have created a demo project, which I have pushed to this repo: https://github.com/galijot/SceneResize-Breaks-UIPageViewController
I have also attached a zip of the project to this report.