Why are my order lines shipped separately instead of being shipped when all lines are available for shipping?

When Allow Partial Shipments of Lines is set to No in Oracle Fusion Cloud Order Management, the order lines are prevented from being split.

But this doesn't prevent lines from being split during shipping. So, in Order Management, set Ship Lines Together to Yes to ship all the order lines together when the order lines become available for shipping.

See the Shipment section of the Oracle Fusion Cloud SCM: Using Order Management guide for details.