[eluser]WanWizard[/eluser]
Nope, sorry, I can't reproduce it here.
I suggest you add some debugging to the session library, using log_message(), and use something you can easy filter out from the rest.
Add a log_message at the beginning of every method, after every assignment to $this->userdata (add 'serialize($this->userdata)' to the logmessage so you know what has been assigned or erased), and before deleting a session record.
Then recreate your problem, and try to find something wrong with the sequence of calls in the log. If you want me to have a look at your (filtered) log, send it to me via PM.
