Hi @floriansebt
I’ve just tested that code on my installation, and it works correctly. Maybe there’s something on your site that prevents the saving process. Could you please try to reproduce the issue on one of the WordPress’ stock themes (like Twenty Sixteen) with other plugins deactivated? If it disappears, then you can activate the theme and plugins one by one to see which one causes the issue.
It’s also possible that you have the max_vars variable issue. In this case, could you please try the solution on this page: https://www.advancedcustomfields.com/resources/limit-number-fields/?
Also, could you please share the JSON or XML export file of your field group so I can check the settings and test it out?
Thanks 🙂
