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Hello,
Thanks for the feedback!
The ACF User field is created & managed by ACF itself (See documentation). ACF Extended doesn’t manage this field.
Regarding your question, you should be able to change which users are selectable using the acf/fields/user/query hook (see documentation). If you need further assistance with this field, I would recommend to contact the ACF Support, they will give you more insights.
Hope it helps!
Have a nice day!
Regards.