In V16 we use templates to send mails from customer sale orders to one of our suppliers.

Although the Default recepients (field use_default_to) is not ticked, To (Emails) field contains the suppliers email and To (Partners) is empty, the email is send to all followers of the sale order including the customer.

Pop-up help for To (Partners) show the help for field partner_to says 
Modifiers: {"invisible":[["use_default_to","=", true]]}
Does this somehow override the setting above for Default recepients (which is not checked)?

How do we avoid odoo sending emails to all followers of the sale order and only send to email addresses in To (Emails) field?