In #13048 session migration was changed so that the old session data are deleted immediately.
The PHP docs in https://secure.php.net/manual/en/features.session.security.management.php state multiple times that this is not the right thing to do, but old session data should be kept for a short while to avoid side effects (concurrency/unstable network).

I did not perceive concrete problems with the current implementation but noted the contradictions between PHP docs and Symfony implementation and wondered if this was done on purpose (although I couldn't find any discussion on that topic).

In short: Can the current implementation be considered correct or good enough or should the regeneration process be improved?