As of macOS Big Sur Beta 8, a potential bug of system UserNotificationCenter.app of AppKit framework could prevent Core Shell/ Tunnel prompt user for passwords/passphrases.
We've filed a bug report to Apple, and there are lots of similar reports already sitting there. Before Apple comes to a conclusion, we recommend you use passphrase-less public-key authentication or ssh-agent to log into your host.
macOS Big Sur Beta 9 seems has fixed bug of system UserNotificationCenter.app of AppKit framework, please upgrade to Beta 9 or later if you're using macOS Big Sur.