Users created from the admin console only store the username when using the Storage Provider SPI

Question

I have created a MongoDbUserStorageProvider to delegate the user storage into MongoDB:

public class MongoDbUserStorageProvider implements
        UserStorageProvider,
        UserLookupProvider,
        CredentialInputValidator,
        CredentialInputUpdater,
        UserQueryProvider,
        UserRegistrationProvider {
        ...
}

public final class MongoUserAdapter extends AbstractUserAdapter {

        ...
    @Override
    public void setFirstName(String firstName) {
        this.userDocument.put("firstName", firstName);
    }
        ...
}

The problem I have is: when I create a user through the Keycloak console, only the username is stored in the database (not the email, firstname, etc.):

As you can see the UserRegistrationProvider.addUser(RealmModel realm, String username) operation only receives the username:

    @Override
    public UserModel addUser(RealmModel realm, String username) {
        final MongoCollection<Document> users = database.getCollection("users");
        Document doc = new Document("username", username);
        users.insertOne(doc);
        Optional<Document> insertedUser = loadUserDocument(new HashMap<String, String>() {{
            put("username", username);
        }});
        return buildUserModel(realm, insertedUser.get());
    }

In org.keycloak.services.resources.admin.UsersResource the method public Response createUser(final UserRepresentation rep) follows this logic:

        try {
            UserModel user = session.users().addUser(realm, rep.getUsername()); //1)
            Set<String> emptySet = Collections.emptySet();

            UserResource.updateUserFromRep(user, rep, emptySet, realm, session, false); //2)
            RepresentationToModel.createCredentials(rep, session, realm, user, true);
            adminEvent.operation(OperationType.CREATE).resourcePath(session.getContext().getUri(), user.getId()).representation(rep).success();

            if (session.getTransactionManager().isActive()) {
                session.getTransactionManager().commit();
            }

            return Response.created(session.getContext().getUri().getAbsolutePathBuilder().path(user.getId()).build()).build();
        } catch (ModelDuplicateException e) {
            if (session.getTransactionManager().isActive()) {
                session.getTransactionManager().setRollbackOnly();
            }
            return ErrorResponse.exists("User exists with same username or email");
        } catch (ModelException me){
            if (session.getTransactionManager().isActive()) {
                session.getTransactionManager().setRollbackOnly();
            }
            logger.warn("Could not create user", me);
            return ErrorResponse.exists("Could not create user");
        }
    }

1) It calls my addUser implementation (storing the new user only with the username)

2) It populates the user object (MongoUserAdapter) with the rest of the fields

However I can't see how to make the populated user object persisted.