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: