When trying to split a sales order line using API, it's throwing an error and not allowing split

When we tried to split a sales order line using API, It's throwing an error which says "You can't update attributes on this order line because part of the line already shipped and it's a return, a remnant, or a line on a sales order that you created before you opted into the update attributes on split order lines for partial shipments feature or the update attributes on split order lines for partial drop shipments feature."

But this line is not yet shipped. Also this order is created on 12th April and we opted into the feature "update attributes on split order lines" on 18th June.