No confimation email when subscribe with the json api

When I use the /subscriptions endpoint of the json api, the new users don’t get their confirmation email.
Under Subscription Configuration the Opt In is set to Double Opt In, Override Opt In is set to false. In the json request: “optin”: “double”.
But no conformation email is set to the users. If I go to the admin page of wordpress, and click resend activation email, they get their actvation email.
Thank you for your help in advance!