I think the confusion comes from the fact that Lookup treats a list of keys as multiple keys to retrieve, rather than treating the whole list as a single key.

For this kind of association, using Key[{1, 2}] makes the intention explicit. Another option is to use assoc[{1, 2}] if you want to access the key directly.

I would not necessarily call the behavior a bug, since Lookup is designed to accept a list when you want to retrieve several keys at once. It just becomes confusing when the keys themselves are lists.

Oh wonderful, thanks!