See: https://twitter.com/hwkfr/status/1101062562198761472

wp-acf-render-field-setting-admin-ajax-fix.php

<?php

add_action('admin_init', 'hwk_acf_render_field_settings_init');
function hwk_acf_render_field_settings_init(){
    $types = acf_get_field_types();
    foreach($types as $type){
        add_action('acf/render_field_settings/type=' . $type->name, 'hwk_acf_render_field_settings', 0);
    }
}

function hwk_acf_render_field_settings($field){
    acf_render_field_setting($field, array(
        'type' 			=> 'text',
        'label' 		=> __('My setting'),
        'instructions'	=> '',
        'name' 			=> 'my_setting',
        'required' 		=> 0,
        'default_value' => ''
    ));
}