Proper sub-domain tracking with Mailchimp LP
Can't you see the correct traffic source when you use Mailchimp landing page?
Mailchimp offers a great feature to collect leads through built-in landing pages. But, you may not reach accurate digital marketing outcomes in Google Analytics reports if you use these pages under your sub-domain.
That is because Mailchimp attaches own UTM parameters to the destination URL. Just switch off this option to fix attribution problem in GA.

(If you can't see the GIF, wait a bit more or click the little icon below.)