The BPS POST Request Attack Protection Bonus Custom Code was being used and required adding an additional whitelist rule for facebook Static HTML iFrame Tabs (highlighted in yellow below).
# BPS POST Request Attack Protection
RewriteCond %{REQUEST_METHOD} POST [NC]
# NEVER COMMENT OUT THIS LINE OF CODE BELOW FOR ANY REASON
RewriteCond %{REQUEST_URI} !^.*/wp-admin/ [NC]
# NEVER COMMENT OUT THIS LINE OF CODE BELOW FOR ANY REASON
RewriteCond %{REQUEST_URI} !^.*/wp-cron.php [NC]
# NEVER COMMENT OUT THIS LINE OF CODE BELOW FOR ANY REASON
RewriteCond %{REQUEST_URI} !^.*/wp-login.php [NC]
# Whitelist the WordPress Theme Customizer
RewriteCond %{HTTP_REFERER} !^.*/wp-admin/customize.php [NC]
# Whitelist XML-RPC Pingbacks, JetPack and Remote Posting POST Requests
RewriteCond %{REQUEST_URI} !^.*/xmlrpc.php [NC]
# Whitelist Network|Multisite Signup POST Form Requests
RewriteCond %{REQUEST_URI} !^.*/wp-signup.php [NC]
# Whitelist Network|Multisite Activate POST Form Requests
RewriteCond %{REQUEST_URI} !^.*/wp-activate.php [NC]
# Whitelist Trackback POST Requests
RewriteCond %{REQUEST_URI} !^.*/wp-trackback.php [NC]
# Whitelist Comments POST Form Requests
RewriteCond %{REQUEST_URI} !^.*/wp-comments-post.php [NC]
# Whitelist facebook Static HTML iframe tabs POST Requests by Query String
RewriteCond %{QUERY_STRING} !^obox-fb= [NC]
RewriteRule ^(.*)$ - [F]