Currently field validation only happens when a post is being published. This is fine when a single user is responsible for creating and publishing posts. For anyone with a publishing workflow, it typically isn’t useful.
For example, a post author might be expected to enter data into some ACF fields and saft as “draft” or a custom post status. Later, an editor tries to publish the post, fails because a field is empty or invalid, but doesn’t have the information to enter the correct data without consulting the author.
Several others have identified this as a useful potential feature:
https://support.advancedcustomfields.com/forums/topic/is-it-possible-to-apply-validation-to-draft-post/
https://support.advancedcustomfields.com/forums/topic/is-validation-possible-on-custom-post-status/
Note that the 3rd party plugin “Validated Field for ACF” is no longer working with current version of WP+ACF Pro.
