- Edited
I've seen this before as well, it can happen if LinkedIn tries to use the "Conditional UI" passkey flow where it asks the browser for a passkey on page load.
On a totally fresh profile for me it doesn't show this, but if you click "Sign in with a passkey" (it appears below Sign in with Apple on my laptop) it does open the security key prompt and doesn't allow me to use a passkey in my Apple Keychain. It should prompt for a passkey same as Safari.
This is on my fresh profile, no way to use a passkey: