Hey Martin,
I checked the code responsible for outputting the dropdown and it looks correct. I’m not sure why it’s passing the condition and outputs the group title. Would you mind removing the group title from Mailchimp.
If that doesn’t work, this would temporarily need custom CSS.

Here’s the CSS that targets the select element of the third fieldset which is your dropdown and moves the select element up covering the label. Add this in Theme Options > CSS
.tco-subscribe-form fieldset:nth-child(5) select {
margin-top: -26px;
}
If you’re going to add fields, you will need to change the nth-child value which is currently 5.
Thanks.