The issue is that the selected value is not available in the choices when the field is loaded and displayed. There needs to be two parts, the JS AJAX part that your using and a PHP part that creates the choices when the field is initially displayed.
This can be accomplished by creating an acf/prepare_field filter. In this filter you can do 1 of 2 things.
I generally opt for #1 because it is faster and easier to do this in php rather than make the extra AJAX request.
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