If you've opened it once on a device and haven't logged out, the encrypted vault is still available on that device and can be unlocked and read. You just can't modify it. There were bugs in the browser extension that made it log out without the user asking it to, but those should be fixed.

I had the Firefox extension log me out yesterday while I had no internet connection.

I'd say if it is a problem being fixed, it is not across the board yet.

In which client? There's no technical requirement for that to be so.

I do find the Firefox browser extension sometimes logs me out (this is separate to the vault lock timer which just asks for a password, the extension basically resets to asking for a user ID)

I've never had that issue in multiple years with spotty internet. What I have is clients that stay out of date and don't always immediately sync. Even when the Internet is fine. Sometimes even a restart wont force a sync.