There is not a lot of documentation on the JS in ACF, I’d do a search of Select2 to see what I could find, but I’m not even sure what to search for, or if there is an action you can trigger in the acf code to force the select2 field to load when the page loads.
Also, you might want to look at the possibility of using a plain old select field that is dynamically loaded with choices https://www.advancedcustomfields.com/resources/dynamically-populate-a-select-fields-choices/
