Having Problems Adding 1Password as Passkey on Login.gov

The US federal government has a website, login.gov, that's intended to provide SSO for other government websites such as the Social Security Administration. login.gov says that it can work with passkeys.

Before 1Password got passkeys out of beta, I set up a passkey using the Apple Keychain. It worked fine. After 1Password came out with passkeys, I deleted the existing passkey from my account at login.gov, and I deleted the passkey itself from the Apple Keychain using macOS Settings > Passwords.

I cannot create a passkey with 1Password. When I ask login.gov to create a new passkey, I get the 1Password "popup" asking if I want to save the passkey in a new or existing login item. I select Existing and click OK. login.gov then shows the error message: "We were unable to add face or touch unlock. Please try again or choose another authentication method."

Any ideas? Thanks.

1Password Version: 8.10.16
Extension Version: 2.15.1
OS Version: macOS 14.0
Browser: Safari 17.0 (19616.1.27.211.1)