It seems that if add the action and POST URL to connect that form to my mailchimp audience, then the form that was just submitted will only send the data to mailchimp - and not send the form to the specified email in Webflow’s native form capabilities.
Is this true, and if so is there a workaround?
Correct, when you set the action, you’re supplying your own form submission handler to replace Webflow’s.
If you really wanted Webflow’s handler to run as well - usually not the case with newsletter enrollments - then you could build an automation against Webflow’s form submission webhook, and add the new subscriber through Mailchimp’s API.