There was a discussion last year (see this thread) whether the HB run-on would continue if there was another reason for the HB to end. Two cases in the guidance were found which suggested the run-on would continue: (1) Moving to another LA, and (2) Becoming a mixed-age couple. In both those cases the guidance is clear that the run-on would still continue.
The run-on for IS is legislated for in precisely the same way as the HB run-on. However, the new ADM Memo is clear that where IS ends for some other reason, the run-on will not continue.
How you reconcile the above, I have no clue. There is certainly room to argue that the IS run-on should also continue for the full 2 weeks.