Update from Miro support [EXPECTED BEHAVIOR]:
After a thorough investigation, the team has identified the reason for the behavior:
Currently, password-protected boards are only shown in the Starred list as long as they are visible to the user.
Once the session expires for the user, the boards become unavailable for them and, as a result, are removed from the Starred list. If the user opens the board via a direct link and re-enters the board password, the board will be shown as Starred again.
That being said, at the moment, this is an expected behavior for password-protected boards.
However, we understand how inconvenient this might be for users needing multiple password-protected boards regularly, and the team is going to consider how the current behavior can be improved. Even though they have plans to work on this, I'm afraid there's no ETA on when the solution will be designed and implemented in the product.