Is this a custom field type that you are creating?
I’m not sure that you can use it where you want to use it as this may require adding custom AJAX calls to return the information, however, I don’t know this at all since I’ve never tried to use a select2 field for a custom field type, you’d have to test it to find out.
I noticed that acf_render_field($field) for a custom field type, doesn’t initialize it into a select2 field with just 'ui' => 1 in the args.
However acf_render_field_wrap($field) DOES automatically initialize select2 when the ui is set to true. The downside here is you have unnecessary nested field wrappers. So I kept digging and all it really boiled down to was needing a simple div with the necessary classes & data attributes. Then ACF is already looking for those and is the ui is also set to a true value… BOOM, select2 is initialized and works great!
Here’s a stripped down code example of it working with a new custom field type: