the goal of _isolate is to temporary get a free view on selected objects and go back per unisolate to the previous state. There is a problem if isolate is used two times.
Example: I have a complex scene and I want to work on two objects of them. I isolate the two objects. Now I want to get only one of them and so I isolate this. If I use unisolate later than I expect to get the state before the last used isolate and I should see the two objects again but I get all objects shown.