Passkey viewable under Passwords but not populating at specific site

Created a passkey for a specific site. I can see the passkey in the Passwords

Yet, when I scan the QR on the site to login, it tells me "There are no matching passkeys saved in your iCloud Keychain for "<site domain>"

I tried airdropping the passkey to my iPad, again I can see the password in Password, but I get the same message. I've triple check the site domain, and it appears exactly the same as in passkey entry, and the message.

I added it again, just incase there was a hidden space or unprintable character. Still getting the same error.

And I can't change settings, because it requires the passkey.

[Re-Titled by Moderator]