I did some checking into this and it has to do with the displaying of the standard fields on the comment form.
When the user is logged in the action is fired before the comment input.
When a user is not logged in the name, email and website fields are shown. The hook will fire after these fields and before the comment input field. However, since by default the name, email and website fields are after the comment field, which is a recent change by WP, the hook does not fire before the comment field and still fires after the name, email and website fields. Until WP changed its behavior the fields in ACF would always appear above the comment field. If you change the order back to the way is used to be then you then it works this way. I found this post that does the trick. http://winaero.com/blog/move-the-comment-text-field-to-the-bottom-in-wordpress-4-4/
I don’t know if this is a bug or not, or even if the developer can do anything about it.
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