Yeah it's happening right now. I'm using build 4.5.8_8411-(8411)
It can be reproduced by opening the app, signing in, opening recents and clearing facer, and then reopening the app and it will have signed me out.
It's been happening for a while now. I assumed it was my phone, but after a factory reset, it was still happening.
UPDATE: Tried clearing app data and cache and logged in again. After reopening the app after force close, I'm still logged in! I'll update this post if I'm logged out in the future.