This sounds like an iOS problem. Bitwarden + Firefox on Linux, or Bitwarden + Android, has never been anything but smooth for me when it comes to saving or using Passkeys.
Bitwarden / ProtonPass + iOS has also been smooth. So sounds like a lastpass issue.
iOS has really good support for third party Password Managers. I've seen the integration with both Strongbox and 1Password. I've heard Bitwarden supports it as well. I think it is a Lastpass problem in this specific case.
If we go for anecdotal examples that are not representative, then in my case, Bitwarden passkey use ends in an endless loop with most websites. Import and export is still a promise or "coming soon." In the case of hardware keys, two devices are needed. I was a proponent of passkeys, but big players screwed that up, like every single shared "standard".