1) I apologise, it is of course on the "Resolutions" tab, not the "Members" tab. The AP Resolution is called "Return a city to their rightful owner", and the UN Resolution is called "U.N. Mandate for city ownership".
2) I think I've got to the bottom of it though. In the Civilopedia, Under BTS Game Concepts>AP and UN, it states (under the AP bit anyway) that cities can only be reassigned from a Voting Member to a Full Member. Under the UN section it just says that all these new concepts are handled in a similar way to the AP. Anyway... since the only way to become a Voting Member in the UN is to defy a resolution, it seems you can only reassign cities from a civ that has recently defied a resolution AND recently captured an enemy city. So that probably explains why it's quite a rare thing to see.