Your comment sounded like you were making a counterpoint, implying that Bitwarden should allow storing of more than one passkey per vault item (if the different passkeys are associated with different “identities” of a single user).
I was simply responding to point out that storing credentials (e.g., passkeys) associated with different “identities” in the same vault login item would not be the best solution.
I had a similar issue and this post got me pretty close to a solution, and I thought I’d share what else I learned in case anyone else had the same problem.
I found this helpful article which suggested that I needed to send both requireResidentKey: true and residentKey: 'required'. My implementation didn’t have residentKey: 'required', and while some other authenticators worked fine without it, BitWarden needs it.