Is this Android? Because, before this change, there was no way to register a real WebAuthn-based passkey with Google, at least when I was trying with chrome (it did not prompt the webauthn popup, just the OS-native security key popup).
At that time, I inspected the registration request Google sent to Chrome and found it was passing a private option that Chrome recognized. According to what I found in web searches for it, the option created a legacy U2F key, and they needed to do that because there were existing Android devices that they could not upgrade and that would not support log-in with WebAuthn keys.