I have actually had the “hide on mobile” code implemented from the start, as the mobile Mailchimp banner is horrendous!
I removed the code to “hide on mobile” and the menu works but unfortunately the popup covers the entire menu.
I have another site running on GeneratePress, and the mobile menu slides out from the left-hand side instead of dropping down. Oddly enough, the MailChimp code works fine on the other site (same version running).. where is the option on GP to switch to a slide-out menu? I can try that.