We are using in our game CreateDraftItem that correctly works with AlternateIds parameter. For UpdateDraftItem, the CatalogItem is the same as the previous API but it always requires the Id assigned by Playfab. This is the error:
"The catalog item must contain a Id. (Parameter 'itemMetadata')"
Looking at the functionality of both endpoints, the case of UpdateDraftItem looks like a bug.
Could you give more info? Thanks!