I have the same issue since Friday. I was able to remove the old owner but I cannot add a new one.
How to change Azure AD Registered Owner?

I'm using the script listed here to change the device owner of an AADJ system in my lab tenant. But I receive the following error when attempting to add a registered owner to that device.
Add-AzureADDeviceRegisteredOwner: Error occurred while executing AddDeviceRegisteredOwner
Code: Request_BadRequest
Message: The URI 'https://graph.windows.net//36d960e0-e339-4a97-a4da-754681fbc0a2/directoryObjects/4582e564-8daa-4992-87a8-8e8c81aca9dd' is not valid since it is not based on 'https://graph.windows.net/36d960e0-e339-4a97-a4da-754681fbc0a2/'.
RequestId: c7467edc-6fae-4cf8-8a2d-512b9ca1de20
DateTimeStamp: Tue, 19 Apr 2022 07:19:57 GMT
HttpStatusCode: BadRequest
HttpStatusDescription: Bad Request
HttpResponseStatus: Completed