When user preference under Setup > Accounting > Accounting Preferences > Order Management > Fulfillment > Manufacturing Issue Based on Commitment = Allow Uncomitted, users are allowed to Build Work Orders that have Buildable Quantity = 0.
Assembly Build will then default to Quantity = 0. When users manually enter the quantity on the Assembly Build and selects the Serial Numbers on the Component Line Items, Serial Numbers shown on the related Work Order does not show up. It is deemed to be committed to the Work Order.
To address this, users must make sure they have enough quantity on the component items to be able to build at least 1. The Inventory Details from the Work Order will then be carried over to the Assembly Build.