Database Sessions losing Userdata?

There's only one way for this to happen - if you've got ajax calls changing the session data (in this case your cart contents), it's possible that one such call is queued to execute after the request that triggers ID regeneration. Thus, the new session data is only written to the old session.

That or a PHP bug, as CI has no control over the regeneration mechanics; it just calls session_regenerate_id().