I received a response from support (thanks, Dennis): only one device can be bound to Hello sign-in due to security-related limitations.
Ref: https://support.lastpass.com/s/document-item?language=en_US&bundleId=lastpass&topicId=LastPass/passwordless_authentication_overview.html&_LANG=enus
Hi, @marc
Welcome to the LastPass Community.
Is this your first time attempting to log in after the password-less set up?
Do you also have an MFA solution synced with your LastPass login?
I am experiencing the same issue. I set up passwordless access some time ago; the passkey is linked to a non-mobile device I own. However, the prompt on my laptop (for a new setup) does not ask to generate a new passkey or bridge between devices and passkeys.