Actually, they would/should. Brave use OSCrypt to encrypt and store passwords securely. OSCrypt relies on the underlying desktop environment’s keyring or keychain service (such as GNOME Keyring or KDE Wallet) to encrypt and decrypt these passwords. When you switch from one desktop environment (like Xfce4) to another (like GNOME), the keyring or keychain service changes. If the new environment does not have access to the old keyring or if the keyring is not unlocked, the passwords will appear to be missing because the encryption keys that OSCrypt relies on are not available.
It might be possible to recover the passwords by either:
Reverting to the original desktop environment or keyring where the passwords were accessible.
Trying to access the keyring from the previous environment using tools or commands that can unlock or export the keyring in the new environment.
However, if the keyring data is not accessible or was overwritten, recovering the encrypted passwords might not be possible.
You should be able to use the --password-store command-line flag with the password store backend Xfce defaults to. Your case is rather interesting, though, as I thought this would be the same as what GNOME defaults to (gnome-libsecret) — can’t verify unfortunately, as I’m AFK for a few days.