Hi @mbastardo.excentia. Hi @Arnaud_Huet and welcome to the community!
Thank you for the report. I can reproduce and confirm the issue.
The fix is done for 10.4.
For now, you can use direct API calls to create ( POST api/users/create) and update ( POST api/users/update) the users. The details of these calls can be found in Help ((?) icon near your avatar at the top right corner) / Web API
Sorry for the inconvenience.
Viktor