I have now tested this, and I can replicate the behaviour with both Vanadium and Brave (after having set the appropriate flag in Brave to enable third-party passkey manager support). Not tested in other browsers.
Steps:
- Have a security key registered for your Bitwarden account
- Make sure Bitwarden is selected as the preferred service in Settings > Passwords, passkeys and autofill
- Clear data for the Bitwarden app (assuming it is already installed and you are signed in)
- Try to sign in to either the Bitwarden app or vault.bitwarden.com/.eu
Observe that you are redirected to the Bitwarden app.
The way I see it, the Chromium autofill implementation is doing exactly what it is designed to do: it is redirecting the user to the preferred passkey manager.
If Play Services had implemented a UI that gave users the additional option of using a security key or a different device, instead of defaulting to on-device and preferred cloud synced passkey managers without any other choice, this behaviour would not occur.