same problem here. I can use the plugin on a multi site, but was not able to get the same fields to appear across sites – I chose the PHP export and then insert into functions.php route, but it actually does not work – the fields are only available in the first sub-site I had set up. Since many (professional ;)) users are maintaining multisites this should be something you care about a bit ;).
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users