I'd like to up this topic. I have the exact same issue, there is a ton of posts about this all across the internet and no clear solution anywhere.
I wonder if it's not about Mailchimp app, they've had quite a lot of sync issues lately according to the reviews on the app store.
We have discontinued using Sumo in favor of utilizing the popup form that MailChimp offers (at no additional cost). I have followed all of the steps in creating the form and publishing it. Because our store is connected, MailChimp does not provide a button to "generate" the code but instead assures Shopify owners that the connection is automatic and the popup form will install and function without any code handling.
However, I have yet to see the actual form on our website. If it's a cookie issue, why doesn't it sppear when opening the website in a new incognito/private window? I am worried the removal of our Sumo form may have been premature and we're going to lose our momentum of steady subscriber additions through our website.
If anyone else experiencing this issue finds the root of the problem, please share with us!
I recommend you to try Personizely
Personizely consolidates a large set of functionality like email popups, exit intent, countdown timer bar, product upsell, cross sell, related products, free shipping bar, sales motivator bar, and website personalization in one place
Also, integrates with MailChimp