It might be that contacts added to your audience via API are not within scope for the double opt-in email: Quote from “About Double Opt-in | Mailchimp”
Although there seems to be an option to ‘mimic’ the invitation via API: Realize Mailchimp Double-Opt-In with API 3.0 - Stack Overflow
So try to set the status of new subscribes to ‘pending’ why adding in Make:
Cheers,
Henk