Because acf uses the same functionality to save its own data that it uses to save our data. Basically the fields you see when editing a field group are the same as the fields you create using them.
There really shouldn’t be any reason that something does not work on localhost that works on a live server. The only difference is the server. So what you need to find out is what is the difference between you local setup and the setup where you host the sites. Start by running phpinfo() on both on both.
Create a file in the root of the site, call it info.php.
<?php phpinfo(); ?>
