possible explanation for this scenario/ behavior, quoting @Andrea Martinez (amt) verbatim from the below post

https://www.odoo.com/forum/help-1/can-i-manage-both-2-step-manufacture-store-and-3-step-pick-manufacture-store-manufacturing-in-odoo-204399

>>>> The reason this is not automatically done by Odoo is that the first rule set on the Manufacture route uses the operation type "Manufacturing". Therefore, when the route is triggered, it uses this rule and this operation type to create the manufacturing order. Since the BOM uses a different operation type, it is not pulled into the MO and does not trigger the transfer needed for the 2-stpe manufacturing process.<<<<

In your business case, do you have multiple routes or multiple warehouses? From your question it seems like the operation type selected on MO wasn't your referred one

I have found the solution : In inventory => Operations Types => I have set Manufacturing / Wood WH before the Manufacturing / San Francisco WH

In the doubt, I have created another product / BoM in San Francisco WH, with Operation Type : "San Francisco: Manufacturing" : the same behavior happens when creating a MO : no default BoM selected :(

Thanks for confirming. This validates the rationale provided in the post, that if the operation type on BOM is different from the 1st rule with operation type manufacture in the DB then the product BOM field on MO needs to be selected manually.

I guess in your use case you were able to change the operation sequence of the manufacturing routes so the preferred operation is chosen first.

The workaround for the MO form, is to override the bom_id field to be required=True.

But I'm still investigating, because I tried a MTO rule and the MO is automatically created without BoM selected (exactly like manually).

My bad : the SO was created using wrong WH. If I set the good WH on the SO, the MO created by the MTO has the good BoM.