Is the front end field an ACF field or your own implementation of Select2?
If it was an ACF field this wouldn’t be an issue, so I’m assuming that it’s the latter.
You would need to get the values in the ACF field and add them to JavaScript loaded by the page to set the selections. I don’t have any examples of this. Basically you’l need to localize your JS to make the values available on the page and then use the values in the Select2 initialization. You’d have to figure out how to do this with Select2.
