the acf/save_post hook can be used https://www.advancedcustomfields.com/resources/acf-save_post/. With a priority < 10 your action will run before new values are saved. You can get the current values of the fields using things like get_field(). You can then look at the values submitted by looking in $_POST['acf'] or you can have another action run with a priority > 10 if you wan to use acf functions like get_field().
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