Membership Renewal Reminder Issue
Turns out Wordfence was blocking the param for the following reason:

blocked by firewall for XSS: Cross Site Scripting in POST body: 
User in city left https://domain.org/wp-admin/admin.php?page=CiviCRM&amp;q=civicrm%2Fadmin%2FscheduleReminders&amp;reset=1 and was blocked by firewall for XSS: Cross Site Scripting in POST body: html_message=%3Cp%3E%26nbsp%3B%3C%2Fp%3E%0A%0A%3Cp%3E%3Cmeta%20charset%3D%22utf-8%22%20%2F%3E%3C%2Fp%3E%0A%0A%3Cp… at https://domain.org/wp-admin/admin.php?page=CiviCRM&amp;q=civicrm%2Fadmin%2FscheduleReminders&amp;action=add&amp;sni…