I am not quite sure how to even begin to troubleshoot this. After some undetermined number of requests or actions an authenticated user will suddenly become anonymous again. Here is a sample series of events with the user status on the left of the arrow and the request they are making on the right
Anonymous -> Home.aspx ... user then clicks the LOGIN button ... Anonymous -> /_layouts/15/Authenticate.aspx Authenticated -> Home.aspx (Note: able to see restricted content) ... several other requests ... ... suddenly falls back to anonymous for no apparent reason ... Anonymous -> AnyPage.aspx
I can't seem to trigger it on demand or through a repeatable sequence. There doesn't seem to be a difference between Publishing pages (customized masterpage) and regular system pages (oslo). Even going to various settings pages will either give the abbreviated settings - lists vanish from All Site Content where permissions hide them from anonymous - or give an error that access is denied (site settings).
Examing Central Admin -> Manage Web Applications -> Authentication Providers shows that I have just one "Claims Based Authentication" provider in the zone "Default". That zone is configured to "Enable anonymous access" and "Enable Windows Authentication / Integrated Windows authentication (NTLM)". FBA and Trusted Identity Providers are not enabled.
Has anyone else encoutered this? Any help or suggestions would be greatly appreciated.