This happens because of how Odoo's procurement logic handles stock allocation vs. MTO demand.

When you build to stock ahead of time and manually allocate later, Odoo sees the component as already available in inventory. Since no new procurement is triggered, no parent-child link is created because the link is established during the procurement generation process, not during manual allocation.

When you use MTO (Make to Order), Odoo creates a fresh procurement that is directly tied to the parent MO, so the relationship is recorded automatically.

The technical reason: Parent-child MO links are created when Odoo runs procurement rules and generates a new MO for a component. If you manually allocate an existing MO to fill a requirement, you are bypassing that procurement generation step, so the system never creates the link.

Solution: If you want the relationship to appear consistently, set your component products to use MTO instead of MTS, or search Odoo Apps for a module that forces parent-child linking on manual allocation.