Very annoying, this is just a bug really. I don´t think you can change it in the email template by using the jinja syntax, there are too many limitations built in. 

So finally i put an additional field in the partner model:

invoice_ids = fields.Char(
    string="Invoice Email", compute="_compute_invoice_email", store=False
)

@api.depends("child_ids.email", "child_ids.type", "email")
def _compute_invoice_email(self):
    for partner in self:
        invoice_contacts = partner.child_ids.filtered(
            lambda c: c.type == "invoice" and c.email
        )
        if invoice_contacts:
            partner.invoice_ids = ",".join(
                [str(id) for id in invoice_contacts.mapped("id")]
            )
        else:
            partner.invoice_ids = ",".join(partner.id)

In the template, you need to point to this field in the right box:

Hi

I don't know odoo 14 (i'm on v16) and don't have a field "invoice address" but as far this is an object from res_partner this should work as email:

(Assuming the invoice child is invoice_partner_id)

 {{ object.invoice_partner_id.id or object.partner_id.id }}

Regards

Hey we have the same question, is anybody here who can help?