Interesting — smells like a bug… 
Update:
This passkey UI is definitely buggy, but the behavior is apparently different in the different browsers. In Chrome, when creating a new passkey for a site (passkeys.io) where I already have four existing passkeys, I see this:
On the one hand, none of the matching passkeys are displayed. On the other hand, I can still add a new passkey using either + New or Save passkey as new login. Seems like those buttons are not present in Safari or Vivaldi, if I look at the screenshots that were posted above/below.
@user48 Here is an oddity that may provide a work-around for you:
Try using the search function on the “Save Passkey” screen (search for anything), but then clear the search term. When I do this in Chrome, the matching login items are now shown on the “Save Passkey” screen:
