Allow for the creation of shipments with more than 200 items through the API

[nighttrain7]

fulfillment_inbound/feeds
In MWS the creation of larger shipments was done by combining shipments through 'UpdateInboundShipment' as seen here: https://docs.developer.amazonservices.com/en_US/fba_guide/FBAGuide_CreateShipment200Items.htmlhttps://docs.developer.amazonservices.com/en_US/fba_guide/FBAGuide_CreateShipment200Items.html
However, it seems the SP_API doesn't allow for that https://github.com/amzn/selling-partner-api-docs/blob/main/references/fulfillment-inbound-api/fulfillmentInboundV0.md#updateinboundshipment "Updates or removes items from the inbound shipment identified by the specified shipment identifier. Adding new items is not supported."

When Using feeds it is possible to create larger shipment plans via POST_FLAT_FILE_FBA_CREATE_INBOUND_PLAN https://developer-docs.amazon.com/sp-api/docs/feed-type-values#fulfillment-by-amazon-feeds
However, there is no function to turn that plan into a real shipment through feeds, and the createInboundShipment operation, which I use for small shipment plans, requires a shipment identifier originally returned by the createInboundShipmentPlan operation (not run when using feeds).
Is this a bug, just something the API is lacking, or am I missing something?
Thank you