Sale order status is Open, but the fulfillment status is Pending...

Recently, we met a problem is -

The status of Sales Order Header and Line is Open, but the fulfillment status of the related line in IN_DEMAND is pending, so that this order cannot be seen by planning, and the line could not be picked up by Order Release Request.

The default status of the Sales Order is Pending, and user needs to update the status to Open when they enter the order. And we didn't have any background process to push the order information to inventory.

We tried many times for different scenarios but cannot replicate the issue. But this case happened sometimes, so I would like to know if anyone know why this happen, and how to avoid it.