Hi guys,
since 2018.3 is finally out, i got to delve a bit deeper into whole improved prefab workflow. When you edit the FSM on the object in the scene you will get the usual "Editing the FSM can disconnect if from the Prefab FSM" dialogue.
Now, when you click EDIT PREFAB, graph view of the FSM will open, the Prefab highlighted in the project view, but you won't be actually editing the Prefab but the INSTANCE since the prefab is not opened, as shown in the screenshot.
I guess you'll get to it eventually, just wanted to let you know.