It was a mess. I think in the end I was able to figure out (with extreme difficulty, because no one at Mailchimp's support was helpful), that it was the pop-up newsletter box I had on my site that was POSSIBLY still connected, since it was added with code and not with a feature. So I removed it. Never happened again.
I haven't tried adding it back in again. Unfortunately the Squarespace support was non very useful: since the box was added with code and not a site feature, they say they can't cover that since it's something I've put manually and not something Squarespace is responsible for.
But yeah, for me it looks like it was the pop up newsletter box. I hope this helps!