When entering a handle before passkey sign-in (eg prefilled by oauth flow), the backend uses start_securitykey_authentication(&credentials) which immediately presents "sign in with a security key" ui and unintentionally bypasses 1Password and other passkey implementations. This means that password manager users are required to delete their handle in order to sign in when authenticating through Oauth.
I'm willing to write and test a PR if that is desired.
A similar issue appears when adding a passkey to an existing account from the Security page, but here there's no option to remove the handle, of course. Happening on Zen 1.21.4b and Firefox 152.0.3.