The current version of ACF PRO (5.4.1) uses Select2 version 3.5.2. The sole reason for this is that it contains more functionality such as the ability to order the selected values. You can check the file in “wp-content/plugins/advanced-custom-fields-pro/assets/inc/select2/select2.js”.
I hope this helps 🙂
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users