Have you killed
yet?
Once you have done that game will trigger everyone who still remains alive to catch up with you in Moonrise Towers, making Isobel leave the confines of Last Light In which in turn should be the triggering point for the cinematic scene you want to see as by that point she becomes inaccessible, you are not doing anything wrong it is just kept very open ended late into Act 2 in case you change mind and want to kill her thus simply telling her that you are not going to do the dirty deed does nothing to resolve as likely intended by design, for the freedom of being able to become a lying backstabber.
Simply take a long rest after you have done the killing of
and caught up with everyone, this is just organically triggered very late into Act 2 if you are not steadfast.
