The session variable 'id_act' is used as part of the URL string when the page redirects. I found that it was picking up the penultimate value for 'id_act', therefore I assumed that the session variable needed to be deleted, so that the latest value for 'id_act' would be used.
I agree that this is unexpected behaviour, so I am going to review my overall process. I suggest that you close this support item, I'll get back to you if I still have a problem.