Hi and welcome to the Testiny forum!
We are not aware of any issue with linking Jira issues as requirements. Maybe it’s some Jira hiccup - can you still reproduce the problem?
If you already linked a specific issue to a test cases, this issue will no longer be shown in the list of Jira issues - but only for that very test case (to not add the same issue as requirement twice), you can still link the same issue as requirement to other test cases.
Noted, we already do have a feature request for this, you might want to vote for that topic?
Best regards,
Alex