In order to create an RFQ by SO when products are configured in MTO, you should follow the following procedure:
Enable multi-step routes in configuration (if not yet done)
Go to the "Buy" route in inventory
Activate debug mode
Select the rule in the "Buy" route and change the field "Propagation of procurement group" from "Leave empty" to "Propagate". You need to do it on the "Buy" route since it is the one generating the RFQs, after being triggered by the MTO route.
Now, the RFQs generated from the MTO route for the same vendor will be grouped together, as long as it is not confirmed into a PO.
Notes:
If you have multiple warehouses, there will be multiple rules in the "Buy" route for which you will have to modify the propagation field.
For dropshipping, the propagate is already set by default: this is because dropshipping would typically require to deliver from different customers, and so it makes no sense to group RFQs together.
Here is a video displaying this:
https://drive.google.com/file/d/1Iye_WkyVfWqLkvA0rjAes1LECMoyiT5R/view