I've tested Proton Pass and a couple more password managers (BitWarden and 1Pass) that should support passkey. I couldn't make them work. Also, I couldn't make Google Password Manager generate a passkey in a profile with Play Services enabled. It might be possible that GrapheneOS or Vanadium do not support passkey. In addition, I've tried different web browsers, such as Brave and Vivaldi and both of them can't be used to generate a passkey.
If someone has managed to generate a passkey via web browser in GrapheneOS I would like to know how.