It seems that Buddypress uses a custom form for the registration form so ACF can’t add its own fields to the form. For something like this, you need to add the fields manually or use the acf_form() function with ‘form’ function set to false. This page should give you more idea about it: https://www.advancedcustomfields.com/resources/acf_form/.
After that, you need to update the newly created user after the registration process by using the update_field() function. Unfortunately, I don’t know which hook you can use. Could you please ask this to Buddypress support?