thats just how Jitsi Meet in a default setup works. Rooms are created on the fly when the first user joins and are destroyed after the last user left. Nothing is stored about the conference on the server side. The list of recent conferences that is shown in the Browser or App is stored in the localStorage, i.e., on the client side.
So if you join the same room again, its actually another room thats created and just happens to be available through the same room name
If you happen to host our own Jitsi deployment, you can try to apply one of the workarounds discussed in this thread to achieve an illusion of persistent rooms/passwords.