In Confluence Cloud:
New pages that are created via Copy Page Tree or Copy Space or even manually with the page editor do not always immediately show up in places where you expect to see pages: in the left navigation page tree, the Children Display macro, in Confluence searches, in the space settings content tools, etc.
This is due to bugs in Confluence’s content caching framework.
Confluence Cloud’s client side code (front-end) is evolving a lot and very quickly. They have migrated more toward a Single Page Application (SPA) front-end which relies heavily on client side caching in your browser. Atlassian has apparently been struggling to keep the SPA data in sync with the back end (cloud service and its persisted data) and to invalidate caches when appropriate. It does seem to be getting better, but there continue to be lots of corner cases where it takes time (or forced refreshes) for the front end to become consistent with the back end.