UPDATE:
As a temporary fix I have put an empty <form></form>
tag before the other forms and now it is working fine. Hope there will be a permanent fix for this issue from a ACF.
UPDATE:
I have multiple ACF forms inside another form (not ACF). If I remove the parent form the first ACF form works again and gets form tags. When adding back the parent form all other ACF forms work, accept the first ACF form. They are all inside the parent form. Is there a solution for this?
I put acf_form_head() now above all html in theme header. That did not solve the issue. Will have another look at the loop code. Will post that here later.