Hmmm, I think I know what's wrong.
If I create the user through the backend (with User role), the form doesn't recognize him, but the command does the trick. However, I want to be able to add users through the backend, why is it failing ? Is it because you can't specify an Authentication provider in the backend ? I can probably configure that in the code though, I'll take a look, thanks !
UPDATE: Ah right, the backend assigns "Neos Backend" as the authentication provider
UPDATE2: Alright got it, I just have to pick the correct authentication provider when creating the user. Question: Is it possible to map an authentication provider based on the user role when the "system default" authentication is picked ?