Sorry for such a late reply on this.
‘wp_head’ is too late to call acf_form_head(). This ACF function must be called before any output is generated for the page. By the time ‘wp_head’ is called I think output has already been generated. It really needs to be done before get_header() or the hook of the same name.
See this reference as it has a list of hooks and their order https://codex.wordpress.org/Plugin_API/Action_Reference.
The only advice I can give is to try an earlier hook.
