As you mentioned that test case icon is not visible, I am able to see from my end.
Could you please clear your cache, cookies and sign out and sign-in Azure DevOps Organization.
Open one of the user stories missing the icon -> Go to the Links tab -> Ensure that test cases are still linked and using the Tests link type. If not, re-link using Add link -> Existing item -> Test case -> Save.
Hope this helps!
Please Let me know if you have any queries.