It appears that a collection relationship field of sub-entities of the same database, the field filter allows a maximum of 2 levels deeper? Deeper levels are not displayed.
I like the field to show a tree of all levels of sub-pages that have the current page as top level.
The filter says “On the list you see only Sub Pages from First Level Page” but that is not true since it shows sub-pages and sub-sub pages, but none on a deeper level. So I don’t understand the logic.