When editing a term or options page, you may find that ACF is not able to save a field’s value. This problem is in most cases easy to diagnose and fix. Please review the following cases for more information.
Field name length
ACF uses the field’s name (see field settings) to save a value into the database. When saving sub fields, the ‘save name’ is generated by the ancestors names + the row index and can become quite long. It is possible that the length of this ‘save name’ can become too long for the wp_options table, and WP will not be able to save the value at all.
Both taxonomy term and options page values are saved into the
wp_options table which has a character limit on the
options_name column of 64 characters.
An example of a nested (x2) sub field could be:
category_1_global_header_images_0_header_image_0_header_image_title which is 67 characters long, or perhaps more if the term ID / row indexes are 10 or higher.
The solution is to either increase the options_name character length of the DB, or reduce the length of your field names.