A footnote belongs to the idea in which you write it in, just as citations do. So that rookery can track them correctly, you need to use the footnote function imported from rookery in ideas, rather than the Typst native function:
#import "@rheo/rookery:0.1.0": idea, footnote
#idea("etal")[
A claim#footnote[The evidence.] worth qualifying.
]Footnote numbering is idea-local. This means that there may be two footnotes labelled 1 on the same page, if two ideas with footnotes are hatched in that context.1
Footnote listings occur at the end of each idea.2 On a standalone page, footnotes appear before the context and backlinks listings.
A footnote written outside an idea’s context proxies the native Typst function so that it behaves normally.