WordPress Custom Fields tab gone after installing ACF Pro plugin

Solution:

Your priority might not be enough. Try like this.

add_filter('acf/settings/remove_wp_meta_box', '__return_false', 20);

EDIT

Please try to initialize ACF with this code

function my_acf_init() {
    acf_update_setting('remove_wp_meta_box', false);
}

add_action('acf/init', 'my_acf_init');