So - found a bug.
Moved a page from one space to another.
The page itself appeared in the new page, but the child pages didn't.
Quick investigation showed that in content manager there is a the main page, but clicking dropdown didn't show any pages.
refreshing page and ticking the box near the main page shows +N pages
clicking dropdown again shows no pages and tick shows +0 pages.
the pages were not orphaned and could be found via MCP
going to the page didn't show it in the UI - but moving it via contextual menu allowed it to be put under main page and show in UI.
Moving 40+ pages like this didn't seem like an option.
Tried seeing child pages via /child-pages block - found that all pages are 2+ levels deep. Meaning skipping something. Found that the skip was folders.
Using Gemini and Claude found a solution - use content -> reorder to move folders (and pages) to re-index them. Folders appeared in UI and pages were visible in the tree too.
Same happened in another space, but not sure if folders were involved or just simply moving it was a problem.
Previous tree structure -> Target tree structure
Content/page/page/folder or page -> Content/page/folder or page
Please see if it's reproducible and if it's a bug.