WordPress Membership Plugin
I recently encountered the same problem. User logged in, went away for a bit, came back and found he was logged in as another user (thus able to access their private info). I do not have any caching plugins installed. I host on FlyWheel, which comes with it’s own caching, but I have disabled it. They do have Fastly CDN by default (which I can’t turn off), could that be causing the issue?