Hi @eddr
It’s possible that the issue is related to your max_vars variable. In this case, kindly try the solution on this page: https://www.advancedcustomfields.com/resources/limit-number-fields/.
It’s also possible that you have a very long field name. Could you please check this page to learn more about it: https://www.advancedcustomfields.com/resources/field-value-wont-save/?
If you want, you can also check the value in your database. It’s saved in the wp_postmeta table. Please don’t forget to backup your database if you decided to check the database.
Another thing you can try is trying 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. This will make sure that the issue is not related to your theme or other plugins.
Thanks 🙂
