Steps to reproduce: (tested on https://demo.strongkey.com/basicdemo or https://demo.strongkey.com/fidopolicy - Minimum-Any-Hardware-Authenticator policy)
1. Register a platform credential with a userid and TouchID (OK)
2. Authenticate with the newly generated credential (OK)
3. Clear browser history (OK)
4. Authenticate with the newly generated credential (Not OK - prompts to login with Security Key)
When using MacBook, macOS Big Sur 11.6, Safari 15: similar results.
When using MacBook, macOS Big Sur 11.6, Google Chrome 80.x: I can successfully authenticate using Platform credentials as long as I do NOT clear "Passwords and other sign-in data" from Advanced tab of "Clear browsing data" - the Basic tab does not delete passwords and other sign-in data.
The Safari UX is a poor one for users who know their userid and where their credential is still available in the site's FIDO Server - that userid can neither be used to register a new Platform credential, nor can it be used to authenticate with the previously registered credential - thus creating a "zombie" credential on the FIDO server.