In odoo/odoo@9c9552fd20 a support of tracking value belonging to another model
than the message has been added. In such cases the formatting method written
at odoo/odoo#124182 fails as it expects a single model for all tracking values
it receives. When field is removed 'model' is False, and we fallback on
mail_message model.
Part-of: odoo/odoo#156571
Alvaro Fuentes
0db60bf1c2