Did you create a normal text field called “INTEREST” ?
That would not fall under the interest group mechanism that is build into Mailchimp and would be overwritten as it’s seen as “just another text field”.
If you are sure this is an interest group, please remove it from the form, update the plugin to the latest version, and then add it again with the button above the form builder.
Interest group code in the form usually looks something like this:
<select name="INTERESTS[f18170f74a]">
<option value="deadb21b06" selected="true">Test group 1</option>
</select>Above the MC4WP form builder there is a separate button to insert interest groups, it looks something like this:
Here is how you create interest groups in your Mailchimp audience:
Once you created a new interest group, in your WordPress admin, go to MC4WP > Mailchimp and click “renew mailchimp lists” to pull in the latest fields and groups from Mailchimp so that you can add them to your form.