"procurement.order" is inherited with a new column sale_line_id in project_mrp.
In action_produce_assign_service, you just have to get the partner assigned to the sale order of the sale_line_id and assign it to the one newly created task_id = project_task.create
Can be easily fixed in 1-line:
"procurement.order" is inherited with a new column sale_line_id in project_mrp.
In action_ produce_ assign_ service, you just have to get the partner assigned to the sale order of the sale_line_id and assign it to the one newly created task_id = project_task.create