1) Create a field group assigned to “menu items”.
2) Include a select field in the group, with “Stylised UI” enabled (Also worth noting that Stylized is apparently spelled wrong in the plugin?)
3) Go to the WordPress menu manager and add a new item to a menu.
Select fields on new menu items will not have the Select2 library initialized on them until the menu is saved and the page is refreshed.
I was alerted to this issue from a user of my ACF FontAwesome plugin which also uses Select2 and is experiencing the same bug. If I find a fix for the problem in my plugin, I will update here with what I find.