Hi Novalis, what you described is exactly my case, same thing. And i´m sure is something related with the acf/save_post… it´s not doing the action when publish posts with gutemberg editor.
As i said, using old Classic editor, works, but for this on WP 5 and up, you need to disable gutemberg.
I´m a little worry about this because i have lot of sites runing wp 4.xxx and i can´t update them.
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users