When FileVault forgets

Sometimes when you boot your MacBook particularly in some recovery modes it will forget the cached FileVault credentials. You then get thrown into a strange mode where it says it forgets and asks for FileVault password. And says you forgot your password. 

Actually it’s not you. It’s the macOS which has lost the encrypted disk credentials

Of course you don’t have it because you never got to type It in. What you need to do is just type in I forgot my password and then you login with your Apple ID. 

It will then recover your FileVault is from iCloud and then ask you to type in a new login password. Then on reboot all will be good.