This seems to work ok, and the new user (non-admin) can see the relevant project when they log in under their ID. However, the project owner is not updated in the “data management” pane of the web interface - the “Owner” box in the table is blank (screenshot).
Thanks for the additional info. What value did you use for the user_id parameter when calling set_project_owner? This needs to be in the form of a user’s MongoDB ID (_id in the users collection). You can query the list of IDs via the MongoDB shell: