I forgot two more steps to reproduce this issue.
Modified steps as follows.
Create a Shortcut to launch and access to https://passkeys-demo.appspot.com/ and place it to desktop on your ios device.
Tap the shortcut to access the url
register account and create passkey.
logoff
access to the url again using the shortcut.
you can see the passkey dialog
unlock device then the dialog disappears
nothing happens
reload the page
proceed 5) to 6)
nothing happens or success webauthn.
When you reload the page, a password dialog may occasionally appear instead of the passkey dialog.