Thank you for posting this in Microsoft Q&A.
I understand that you are trying to create a user in Azure B2C using Graph Client (5.40.0), but you are getting a response saying that "userPrincipalName already exists". When you try to get the user using the user's principal name, you are getting an error code "Request_ResourceNotFound".
Could you attempt to retrieve the user by utilizing their object ID and observe the outcome?
Please follow the document https://learn.microsoft.com/en-us/graph/api/user-post-users?view=graph-rest-1.0&tabs=csharp#example-1-create-a-user to create user using graph client.
For your reference: User management in B2C
If you are still encountering issue, please share the Graph Client request that you use to create a user and obtain their information?
Hope this helps. Do let us know if you any further queries.
Thanks,
Navya.