This fixed https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/32313185
and this only happened on MySQL, because without this, the user
would have another namespace, and both namespaces are belonging
to the user here. However user.namespace would only return one
namespace, which one is database dependent. Even worse, since
there's no ORDER BY, it should be considered completely random.

We should refactor User#ensure_namespace_correct in the future.