Official comment
WPEngine is a popular, premium WordPress host that includes a power server-side cache system they call Evercache.
Lately, they have also been using other caching techniques, including drop in plugins for caching.
All caches serve the page from memory without checking to see if the logged in user has permission to see the page. Non-members can view the page this way, but often PilotPress will jump into action after the fact and try to redirect the non-member away. Often this results in the login box appearing on the protected page, or a valid member being redirected back to the original log in page.
Until recently, WPEngine support was very helpful in excluding pages from their proprietary cache system. Recently they have stopped being as helpful, and several of our clients have had to move away from their proprietary hosting platform.
We recommend using another web host for your site if WPEngine is unable to disable all cache from your membership pages.
