I would not be surprised if the Linode "reset root password" function shuts down your VM, mounts the filesystem directly on the host, and edits /etc/shadow, and

I would not be surprised if the Linode "reset root password" function shuts down your VM, mounts the filesystem directly on the host, and edits /etc/shadow, and maybe the PAM configs if they're feeling real nice. Using something we can't mount (e.g. encrypted)? Not using /etc/shadow nor PAM? Sorry, we can't help you beyond advising you try and reboot in single user and ssh in to your VM console.