Hi @svenni
My previous comment was mean to say: Are you sure the <!--nextpage--> exists.
I now understand that this is part of WP, so please ignore that question.
I had a read about the custom query but this again is basing the ‘pages’ on what is in the_content, not what is in a custom field.
Are you able to ask the developer of the pageNavi plugin, if it is possible to use his plugin with an ACF WYSIWYG field?
Cheers
E
