Laravel 5.2 sessions resetting

I have a form where the session and CSRF token reset at seemingly random intervals, although it's probably not random, since it only happens on this one form. I can edit and save the data several times in succession with no problem, then after a while a redirect()->back() results in a CSRF mismatch. At that point the CSRF token and session cookies show that they are brand new, but this is far short of 2 hours.

I have set auth:login and auth:logout event listeners to write to the logs, and they are not firing.

I'm looking for ideas on how I can diagnose this and determine where this is happening. I"m not using any session()->puts or any other session helpers or facades.

Thanks.

via Chebli Mohamed