Cannot opt-in a previously opted-out user via api

We have a Keap integration where users fill out a form and enter a promotion and we pass the contact data to Keap.

If our client sends an email to the user and that user decides to unsubscribe from emails they are opted-out.

Let’s say down the road that same user then decided to sign up for another promotion. When we attempt to pass their data back into Keap, their status remains as opted-out, even though they opted-in again by filling out another form.

How do we change this users status via the api?

This is an intentionally prohibited action via the API, as it would be simple to just programmatically flip every Contact to Marketable whenever they opted out, putting our deliverability rating at risk and violating local ordinances.