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No, it’s not normal for an API key to disappear. Are you sure you are logged in to the same Mailchimp account?
In your WordPress admin, under MC4WP > Other, can you set it to “log everything”, then test the form, and then check the log. Does it show a success message in the log? Then the form is working properly (meaning the API key still exists).
If under MC4WP > Form > Settings double opt-in is turned on, the subscriber won’t show up in the Mailchimp list until the confirmation link is clicked in the email.
Hope that helps. If you have any questions, please let me know!