As far as I know, there isn’t a way to fire the save. ACF is fired to save when a post is saved or a term and there is something custom in options pages, but I think it still happens on a WP hook.
What you’ll need to do is check do a check of the existing rows when a row is updated and create the ID for rows where it is missing.
PS: There are other people that answer questions. I’m as active as I am because, well, I am paid help, or give what help I can. I can’t answer every question because some of them are beyond my knowledge. This is a user forum and depends on users. In the end I am really just another user, I have been using ACF for so long that I can’t remember building a site without it and I also like helping others if I can. Other people like me have come and gone, it’s not an easy job and can be frustrating at times.
