Without further information, this sounds like the way the add-on was scripted.
Unless I could see the code I'm not sure what to suggest other than suggesting you speak to the add-on creator and asking them to put in a line to prevent the Slides returning to slide 1.
I assume that it is a private add-on that you are speaking of rather than one that can be downloaded by anyone?
Of course, I could be wrong and it could be a limitation of Slides but I don't remember the film strip moving by itself on any of my Slides add-ons.