This is just a guess, but it’s what I think is going on. Since the select2 field is loaded using AJAX then somehow WP is blocking the ACF from getting the list of terms from the taxonomy if the user is not logged in.
At any rate, something is preventing the AJAX request from completing.
That being said, I just tried a taxonomy field, on a font end form using acf_form(), while logged out and it works perfectly. This is using current version of WP, 2017 them, ACF Pro and no other plugins and testing using the built in “category” taxonomy.
