GraphQL update... mutation for Fabric SQL DB failing

Hi everybody, 
I want to update items in a Fabric SQL DB via GraphQL mutation query. But that fails with "InternalServerError" (and nothing else). 
What I tried: 

• ❌ mutation update....
• ✅ mutation create...
• ✅ mutation delete...

• ✅ (directly in the SQL DB): UPDATE dbo.product SET product_name = 'CHANGED' WHERE ROWID = '<UUID>'

Did someone else here experience something similar? Is this a defect 🐞 in the GraphQL API / Fabric SQL DB / their combination?
What was your solution? (And what was the issue exactly?)
A workaround might be to create a stored proc and let that do the update. I'd prefer to be able to use a working update mutation though. So

Any help highly appreciated! <3 --> Snippets below

Best
Martin

(Snippets stripped down for readability) 
The failing mutation: 

mutation {
    updateproduct (
        ROWID: "<UUID>", 
        item: {
            product_name: "CHANGED"
        }) 
        {
            ROWID product_name
        }
}

//// Definitions from the GraphQL schema: 
"""Updates a product"""
  updateproduct (
    """The ID of the item being updated."""
    ROWID: UUID!
    """Input representing all the fields for updating product"""
    item: UpdateproductInput!
  ): product

"""Input type for updating product_mapping"""
input UpdateproductInput {
  """Input for field ROWID on type UpdateproductInput"""
  ROWID: UUID

  """Input for field product_name on type UpdateproductInput"""
  product_name: String
}

The table in Fabric SQL DB (as by "Script as create")

CREATE TABLE [dbo].[product](
	[ROWID] [uniqueidentifier] NOT NULL,
	[product_name] [nvarchar](255) NOT NULL,
) ON [PRIMARY]

ALTER TABLE [dbo].[product_mapping]
  ADD PRIMARY KEY CLUSTERED (
	[ROWID] ASC
  ) WITH ( STATISTICS_NORECOMPUTE = OFF,  IGNORE_DUP_KEY = OFF, 
                ONLINE = OFF, OPTIMIZE_FOR_SEQUENTIAL_KEY = OFF)
ON [PRIMARY]

ALTER TABLE [dbo].[product_mapping]
  ADD  DEFAULT (newsequentialid()) FOR [ROWID]