I used Firefox (v74) on Mac to register a FIDO credential with WebAuthn using a couple of different popular USB authenticators (Yubikey 5 and an eWBM GoldenGate). I understand Firefox is probably using CTAP1, and if I use attestation I noticed that a fido-u2f attestation format is returned. Regardless, a public key credential is provided and saved against the user account. Later, using either generally available Safari 13.0.5 or Safari Tech Preview Release 101, when I try to use this credential in an allowCredentials list to perform an assertion flow, Safari reports in a dialog: "Found no credentials on this device" and I am unable to complete assertion. Both the original FF browser, and Chrome on the same Mac are able to use the credential to complete assertion.
It appears I can actually complete authentication - that part is false in my original statement - however the error message displayed in the dialog is still something that should be addressed.
Actually it doesn't matter what browser is used when the credential was registered - even on Safari itself. This msg "Found no credentials on this device" is displayed all the time. Seems like an odd message to display just because a roaming authenticator was used.
<rdar://problem/59921962>
It doesn't reproduce for me on ToT WebKit. Could you try the latest STP again? It will be better if you could specify the exact website you are using and upload a video.