To reproduce the problem (example):
- install the project app with demo data
- activate the debug mode
- go to Settings -> Technical -> Templates
- choose the template "Task: Rating Request"
- in email configuration, "To (partners)" is set to
${object.rating_get_partner_id().id}
- click on preview
- choose the task: "Room 2: Decoration"
- the "Recipients" field value should be "YourCompany, Joel Willis" but this
field remains empty without the fix

Technical note: partner_to of the template was not rendered which was making
generate_recipients returning incorrect values for partner_ids.

Task-3196081

Part-of: odoo/odoo#113341