Hi @FrankatMSC,
I have just checked this and updating the sponsor id indeed assigns the Project sponsor. Here is the screenshot of my update module

Could you please verify if you are using correct connection (for example we had issues in the past where someone used preview connection instead of production)? Is the project ID correct and also is the sponsor ID correct?
Also to clarify I have tried the same approach with updating the Project Sponsor once there is one already assigned, and it also worked.
Thank you in advance.
Best regards,
Ivan