The API documentation is still on version 1.0.0 (which I find hard to believe after a few years) - https://api.parcel.royalmail.com/

We were able to create a shipment with multiple parcels in it (as per the docs) but now it seems we are unable to. There was also a bug where the same SKU could not be in two different parcels within the same shipment (so perhaps this was the solution)

Please please please update the Click & Drop API documentation, and notify anyone using this API of forthcoming changes before they are implemented as per good practise