I took a look at it. The Subscription page uses the <p>{subscription_form}</p> code.
In the SUBSCRIPTION FORM FIELDS AND LAYOUT > Main profile fields, there is nothing special
https://www.vekn.net/tmp/subplugin1.png
However, the generated form has an extra field called ‘tnp-lists’ that has no options
<form method="post" action="https://www.blackchantry.com/?na=s">
<input type="hidden" name="nr" value="widget"><input type="hidden" name="nlang" value=""><div class="tnp-field tnp-field-firstname"><label for="tnp-name">First name</label>
<input class="tnp-name" type="text" name="nn" value="" required=""></div>
<div class="tnp-field tnp-field-surname"><label for="tnp-surname">Last name</label>
<input class="tnp-surname" type="text" name="ns" value="" required=""></div><div class="tnp-field tnp-field-email"><label for="tnp-email">Email</label>
<input class="tnp-email" type="email" name="ne" value="" required=""></div>
<div class="tnp-field tnp-field-gender"><label for="tnp-sex">I'm</label>
<select name="nx" class="tnp-gender" id="tnp-gender" required=""><option value=""></option><option value="n">Not specified</option><option value="f">Woman</option><option value="m">Man</option></select></div>
<div class="tnp-field tnp-lists"><select class="tnp-lists" name="nl[]" required=""></select></div><div class="tnp-field tnp-field-profile">...</div><div class="tnp-field tnp-field-button"><input class="tnp-submit" type="submit" value="Subscribe">
</div>
</form>
https://www.vekn.net/tmp/subplugin2.png