Coises, automatically deleting the local copy of the vault data when you log out of the app is an important security feature. You don’t want to be leaving encrypted copies of your vault on every device you ever logged into a Bitwarden app on. Knowing your master password is the only thing stopping a person from being able to access your vault data if they can access the app’s data file on the device. They don’t need a second factor, and access can’t be revoked remotely if the device is offline, since Bitwarden is designed to allow read-only access even when offline.
I believe you can back up the local encrypted vault data, and restore it to get to an old version of the vault, or to restore access in the event that you’ve logged out and the Bitwarden cloud service is unavailable. I tested it with the Firefox app, and it seems to work fine. If you log out of the app, restore a data file backed up when you were logged in, and then restart the app, you’ll be logged in again and you only need to enter your master password to unlock the vault. This works without an Internet connection. A far as I know, the app login never expires. But you will need to remember the master password you used at the time of the backup, because entering that password is the only way to unlock and decrypt the vault.
Being able to create automatic backups of my vault is an important feature to me, so I can guard against corruption of the vault data, see my old passwords if necessary, and ensure access to my vault even if the Bitwarden cloud service is unavailable.